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<li><a class="reference internal" href="#">What’s New In Python 3.8</a><ul>
<li><a class="reference internal" href="#summary-release-highlights">Summary – Release highlights</a></li>
<li><a class="reference internal" href="#new-features">New Features</a><ul>
<li><a class="reference internal" href="#assignment-expressions">Assignment expressions</a></li>
<li><a class="reference internal" href="#positional-only-parameters">Positional-only parameters</a></li>
<li><a class="reference internal" href="#parallel-filesystem-cache-for-compiled-bytecode-files">Parallel filesystem cache for compiled bytecode files</a></li>
<li><a class="reference internal" href="#debug-build-uses-the-same-abi-as-release-build">Debug build uses the same ABI as release build</a></li>
<li><a class="reference internal" href="#f-strings-support-for-self-documenting-expressions-and-debugging">f-strings support <code class="docutils literal notranslate">=</code> for self-documenting expressions and debugging</a></li>
<li><a class="reference internal" href="#pep-578-python-runtime-audit-hooks">PEP 578: Python Runtime Audit Hooks</a></li>
<li><a class="reference internal" href="#pep-587-python-initialization-configuration">PEP 587: Python Initialization Configuration</a></li>
<li><a class="reference internal" href="#pep-590-vectorcall-a-fast-calling-protocol-for-cpython">PEP 590: Vectorcall: a fast calling protocol for CPython</a></li>
<li><a class="reference internal" href="#pickle-protocol-5-with-out-of-band-data-buffers">Pickle protocol 5 with out-of-band data buffers</a></li>
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<li><a class="reference internal" href="#api-and-feature-removals">API and Feature Removals</a></li>
<li><a class="reference internal" href="#porting-to-python-3-8">Porting to Python 3.8</a><ul>
<li><a class="reference internal" href="#changes-in-python-behavior">Changes in Python behavior</a></li>
<li><a class="reference internal" href="#changes-in-the-python-api">Changes in the Python API</a></li>
<li><a class="reference internal" href="#changes-in-the-c-api">Changes in the C API</a></li>
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<li><a class="reference internal" href="#notable-changes-in-python-3-8-1">Notable changes in Python 3.8.1</a></li>
<li><a class="reference internal" href="#notable-changes-in-python-3-8-8">Notable changes in Python 3.8.8</a></li>
<li><a class="reference internal" href="#notable-changes-in-python-3-8-12">Notable changes in Python 3.8.12</a></li>
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 <section id="what-s-new-in-python-3-8">
<h1>What’s New In Python 3.8<a class="headerlink" href="#what-s-new-in-python-3-8" title="Permalink to this headline">¶</a></h1>
<dl class="field-list simple">
<dt class="field-odd">Editor</dt>
<dd class="field-odd">Raymond Hettinger
</dd>
</dl>
This article explains the new features in Python 3.8, compared to 3.7.
Python 3.8 was released on October 14, 2019.
For full details, see the <a class="reference internal" href="changelog.html#changelog">changelog</a>.
<section id="summary-release-highlights">
<h2>Summary – Release highlights<a class="headerlink" href="#summary-release-highlights" title="Permalink to this headline">¶</a></h2>
</section>
<section id="new-features">
<h2>New Features<a class="headerlink" href="#new-features" title="Permalink to this headline">¶</a></h2>
<section id="assignment-expressions">
<h3>Assignment expressions<a class="headerlink" href="#assignment-expressions" title="Permalink to this headline">¶</a></h3>
There is new syntax <code class="docutils literal notranslate">:=</code> that assigns values to variables as part of a larger
expression. It is affectionately known as “the walrus operator” due to
its resemblance to <a class="reference external" href="https://en.wikipedia.org/wiki/Walrus#/media/File:Pacific_Walrus_-_Bull_(8247646168).jpg">the eyes and tusks of a walrus</a>.
In this example, the assignment expression helps avoid calling
<a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate">len()</code></a> twice:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>if (n := len(a)) &gt; 10:
 print(f&quot;List is too long ({n} elements, expected &lt;= 10)&quot;)
</pre></div>
</div>
A similar benefit arises during regular expression matching where
match objects are needed twice, once to test whether a match
occurred and another to extract a subgroup:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>discount = 0.0
if (mo := re.search(r&#39;(\d+)% discount&#39;, advertisement)):
 discount = float(mo.group(1)) / 100.0
</pre></div>
</div>
The operator is also useful with while-loops that compute
a value to test loop termination and then need that same
value again in the body of the loop:
<div class="highlight-python3 notranslate"><div class="highlight"><pre># Loop over fixed length blocks
while (block := f.read(256)) != &#39;&#39;:
 process(block)
</pre></div>
</div>
Another motivating use case arises in list comprehensions where
a value computed in a filtering condition is also needed in
the expression body:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>[clean_name.title() for name in names
 if (clean_name := normalize(&#39;NFC&#39;, name)) in allowed_names]
</pre></div>
</div>
Try to limit use of the walrus operator to clean cases that reduce
complexity and improve readability.
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0572">PEP 572</a> for a full description.
(Contributed by Emily Morehouse in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35224">bpo-35224</a>.)
</section>
<section id="positional-only-parameters">
<h3>Positional-only parameters<a class="headerlink" href="#positional-only-parameters" title="Permalink to this headline">¶</a></h3>
There is a new function parameter syntax <code class="docutils literal notranslate">/</code> to indicate that some
function parameters must be specified positionally and cannot be used as
keyword arguments. This is the same notation shown by <code class="docutils literal notranslate">help()</code> for C
functions annotated with Larry Hastings’
<a class="reference internal" href="../howto/clinic.html#howto-clinic">Argument Clinic</a> tool.
In the following example, parameters a and b are positional-only,
while c or d can be positional or keyword, and e or f are
required to be keywords:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def f(a, b, /, c, d, *, e, f):
 print(a, b, c, d, e, f)
</pre></div>
</div>
The following is a valid call:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>f(10, 20, 30, d=40, e=50, f=60)
</pre></div>
</div>
However, these are invalid calls:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>f(10, b=20, c=30, d=40, e=50, f=60) # b cannot be a keyword argument
f(10, 20, 30, 40, 50, f=60) # e must be a keyword argument
</pre></div>
</div>
One use case for this notation is that it allows pure Python functions
to fully emulate behaviors of existing C coded functions. For example,
the built-in <a class="reference internal" href="../library/functions.html#divmod" title="divmod"><code class="xref py py-func docutils literal notranslate">divmod()</code></a> function does not accept keyword arguments:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def divmod(a, b, /):
 &quot;Emulate the built in divmod() function&quot;
 return (a // b, a % b)
</pre></div>
</div>
Another use case is to preclude keyword arguments when the parameter
name is not helpful. For example, the builtin <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate">len()</code></a> function has
the signature <code class="docutils literal notranslate">len(obj, /)</code>. This precludes awkward calls such as:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>len(obj=&#39;hello&#39;) # The &quot;obj&quot; keyword argument impairs readability
</pre></div>
</div>
A further benefit of marking a parameter as positional-only is that it
allows the parameter name to be changed in the future without risk of
breaking client code. For example, in the <a class="reference internal" href="../library/statistics.html#module-statistics" title="statistics: Mathematical statistics functions"><code class="xref py py-mod docutils literal notranslate">statistics</code></a> module, the
parameter name dist may be changed in the future. This was made
possible with the following function specification:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def quantiles(dist, /, *, n=4, method=&#39;exclusive&#39;)
 ...
</pre></div>
</div>
Since the parameters to the left of <code class="docutils literal notranslate">/</code> are not exposed as possible
keywords, the parameters names remain available for use in <code class="docutils literal notranslate">**kwargs</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def f(a, b, /, **kwargs):
... print(a, b, kwargs)
...
&gt;&gt;&gt; f(10, 20, a=1, b=2, c=3) # a and b are used in two ways
10 20 {&#39;a&#39;: 1, &#39;b&#39;: 2, &#39;c&#39;: 3}
</pre></div>
</div>
This greatly simplifies the implementation of functions and methods
that need to accept arbitrary keyword arguments. For example, here
is an excerpt from code in the <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate">collections</code></a> module:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class Counter(dict):

def __init__(self, iterable=None, /, **kwds):
 # Note &quot;iterable&quot; is a possible keyword argument
</pre></div>
</div>
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0570">PEP 570</a> for a full description.
(Contributed by Pablo Galindo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36540">bpo-36540</a>.)
</section>
<section id="parallel-filesystem-cache-for-compiled-bytecode-files">
<h3>Parallel filesystem cache for compiled bytecode files<a class="headerlink" href="#parallel-filesystem-cache-for-compiled-bytecode-files" title="Permalink to this headline">¶</a></h3>
The new <a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPYCACHEPREFIX"><code class="xref std std-envvar docutils literal notranslate">PYTHONPYCACHEPREFIX</code></a> setting (also available as
<a class="reference internal" href="../using/cmdline.html#cmdoption-X"><code class="xref std std-option docutils literal notranslate">-X</code></a> <code class="docutils literal notranslate">pycache_prefix</code>) configures the implicit bytecode
cache to use a separate parallel filesystem tree, rather than
the default <code class="docutils literal notranslate">__pycache__</code> subdirectories within each source
directory.
The location of the cache is reported in <a class="reference internal" href="../library/sys.html#sys.pycache_prefix" title="sys.pycache_prefix"><code class="xref py py-data docutils literal notranslate">sys.pycache_prefix</code></a>
(<a class="reference internal" href="../library/constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate">None</code></a> indicates the default location in <code class="docutils literal notranslate">__pycache__</code>
subdirectories).
(Contributed by Carl Meyer in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33499">bpo-33499</a>.)
</section>
<section id="debug-build-uses-the-same-abi-as-release-build">
<h3>Debug build uses the same ABI as release build<a class="headerlink" href="#debug-build-uses-the-same-abi-as-release-build" title="Permalink to this headline">¶</a></h3>
Python now uses the same ABI whether it’s built in release or debug mode. On
Unix, when Python is built in debug mode, it is now possible to load C
extensions built in release mode and C extensions built using the stable ABI.
Release builds and <a class="reference internal" href="../using/configure.html#debug-build">debug builds</a> are now ABI compatible: defining the
<code class="docutils literal notranslate">Py_DEBUG</code> macro no longer implies the <code class="docutils literal notranslate">Py_TRACE_REFS</code> macro, which
introduces the only ABI incompatibility. The <code class="docutils literal notranslate">Py_TRACE_REFS</code> macro, which
adds the <code class="xref py py-func docutils literal notranslate">sys.getobjects()</code> function and the <a class="reference internal" href="../using/cmdline.html#envvar-PYTHONDUMPREFS"><code class="xref std std-envvar docutils literal notranslate">PYTHONDUMPREFS</code></a>
environment variable, can be set using the new <a class="reference internal" href="../using/configure.html#cmdoption-with-trace-refs"><code class="xref std std-option docutils literal notranslate">./configure
--with-trace-refs</code></a> build option.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36465">bpo-36465</a>.)
On Unix, C extensions are no longer linked to libpython except on Android
and Cygwin.
It is now possible
for a statically linked Python to load a C extension built using a shared
library Python.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=21536">bpo-21536</a>.)
On Unix, when Python is built in debug mode, import now also looks for C
extensions compiled in release mode and for C extensions compiled with the
stable ABI.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36722">bpo-36722</a>.)
To embed Python into an application, a new <code class="docutils literal notranslate">--embed</code> option must be passed to
<code class="docutils literal notranslate">python3-config --libs --embed</code> to get <code class="docutils literal notranslate">-lpython3.8</code> (link the application
to libpython). To support both 3.8 and older, try <code class="docutils literal notranslate">python3-config --libs
--embed</code> first and fallback to <code class="docutils literal notranslate">python3-config --libs</code> (without <code class="docutils literal notranslate">--embed</code>)
if the previous command fails.
Add a pkg-config <code class="docutils literal notranslate">python-3.8-embed</code> module to embed Python into an
application: <code class="docutils literal notranslate">pkg-config python-3.8-embed --libs</code> includes <code class="docutils literal notranslate">-lpython3.8</code>.
To support both 3.8 and older, try <code class="docutils literal notranslate">pkg-config python-X.Y-embed --libs</code> first
and fallback to <code class="docutils literal notranslate">pkg-config python-X.Y --libs</code> (without <code class="docutils literal notranslate">--embed</code>) if the
previous command fails (replace <code class="docutils literal notranslate">X.Y</code> with the Python version).
On the other hand, <code class="docutils literal notranslate">pkg-config python3.8 --libs</code> no longer contains
<code class="docutils literal notranslate">-lpython3.8</code>. C extensions must not be linked to libpython (except on
Android and Cygwin, whose cases are handled by the script);
this change is backward incompatible on purpose.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36721">bpo-36721</a>.)
</section>
<section id="f-strings-support-for-self-documenting-expressions-and-debugging">
<h3>f-strings support <code class="docutils literal notranslate">=</code> for self-documenting expressions and debugging<a class="headerlink" href="#f-strings-support-for-self-documenting-expressions-and-debugging" title="Permalink to this headline">¶</a></h3>
Added an <code class="docutils literal notranslate">=</code> specifier to <a class="reference internal" href="../glossary.html#term-f-string">f-string</a>s. An f-string such as
<code class="docutils literal notranslate">f'{expr=}'</code> will expand to the text of the expression, an equal sign,
then the representation of the evaluated expression. For example:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; user = &#39;eric_idle&#39;
&gt;&gt;&gt; member_since = date(1975, 7, 31)
&gt;&gt;&gt; f&#39;{user=} {member_since=}&#39;
&quot;user=&#39;eric_idle&#39; member_since=datetime.date(1975, 7, 31)&quot;
</pre></div>
</div>
The usual <a class="reference internal" href="../reference/lexical_analysis.html#f-strings">f-string format specifiers</a> allow more
control over how the result of the expression is displayed:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; delta = date.today() - member_since
&gt;&gt;&gt; f&#39;{user=!s} {delta.days=:,d}&#39;
&#39;user=eric_idle delta.days=16,075&#39;
</pre></div>
</div>
The <code class="docutils literal notranslate">=</code> specifier will display the whole expression so that
calculations can be shown:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; print(f&#39;{theta=} {cos(radians(theta))=:.3f}&#39;)
theta=30 cos(radians(theta))=0.866
</pre></div>
</div>
(Contributed by Eric V. Smith and Larry Hastings in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36817">bpo-36817</a>.)
</section>
<section id="pep-578-python-runtime-audit-hooks">
<h3>PEP 578: Python Runtime Audit Hooks<a class="headerlink" href="#pep-578-python-runtime-audit-hooks" title="Permalink to this headline">¶</a></h3>
The PEP adds an Audit Hook and Verified Open Hook. Both are available from
Python and native code, allowing applications and frameworks written in pure
Python code to take advantage of extra notifications, while also allowing
embedders or system administrators to deploy builds of Python where auditing is
always enabled.
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0578">PEP 578</a> for full details.
</section>
<section id="pep-587-python-initialization-configuration">
<h3>PEP 587: Python Initialization Configuration<a class="headerlink" href="#pep-587-python-initialization-configuration" title="Permalink to this headline">¶</a></h3>
The <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0587">PEP 587</a> adds a new C API to configure the Python Initialization
providing finer control on the whole configuration and better error reporting.
New structures:
<ul class="simple">
<li><a class="reference internal" href="../c-api/init_config.html#c.PyConfig" title="PyConfig"><code class="xref c c-type docutils literal notranslate">PyConfig</code></a></li>
<li><a class="reference internal" href="../c-api/init_config.html#c.PyPreConfig" title="PyPreConfig"><code class="xref c c-type docutils literal notranslate">PyPreConfig</code></a></li>
<li><a class="reference internal" href="../c-api/init_config.html#c.PyStatus" title="PyStatus"><code class="xref c c-type docutils literal notranslate">PyStatus</code></a></li>
<li><a class="reference internal" href="../c-api/init_config.html#c.PyWideStringList" title="PyWideStringList"><code class="xref c c-type docutils literal notranslate">PyWideStringList</code></a></li>
</ul>
New functions:
<ul class="simple">
<li><code class="xref c c-func docutils literal notranslate">PyConfig_Clear()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyConfig_InitIsolatedConfig()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyConfig_InitPythonConfig()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyConfig_Read()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyConfig_SetArgv()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyConfig_SetBytesArgv()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyConfig_SetBytesString()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyConfig_SetString()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyPreConfig_InitIsolatedConfig()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyPreConfig_InitPythonConfig()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyStatus_Error()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyStatus_Exception()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyStatus_Exit()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyStatus_IsError()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyStatus_IsExit()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyStatus_NoMemory()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyStatus_Ok()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyWideStringList_Append()</code></li>
<li><code class="xref c c-func docutils literal notranslate">PyWideStringList_Insert()</code></li>
<li><a class="reference internal" href="../c-api/veryhigh.html#c.Py_BytesMain" title="Py_BytesMain"><code class="xref c c-func docutils literal notranslate">Py_BytesMain()</code></a></li>
<li><code class="xref c c-func docutils literal notranslate">Py_ExitStatusException()</code></li>
<li><a class="reference internal" href="../c-api/init_config.html#c.Py_InitializeFromConfig" title="Py_InitializeFromConfig"><code class="xref c c-func docutils literal notranslate">Py_InitializeFromConfig()</code></a></li>
<li><a class="reference internal" href="../c-api/init_config.html#c.Py_PreInitialize" title="Py_PreInitialize"><code class="xref c c-func docutils literal notranslate">Py_PreInitialize()</code></a></li>
<li><a class="reference internal" href="../c-api/init_config.html#c.Py_PreInitializeFromArgs" title="Py_PreInitializeFromArgs"><code class="xref c c-func docutils literal notranslate">Py_PreInitializeFromArgs()</code></a></li>
<li><a class="reference internal" href="../c-api/init_config.html#c.Py_PreInitializeFromBytesArgs" title="Py_PreInitializeFromBytesArgs"><code class="xref c c-func docutils literal notranslate">Py_PreInitializeFromBytesArgs()</code></a></li>
<li><a class="reference internal" href="../c-api/init_config.html#c.Py_RunMain" title="Py_RunMain"><code class="xref c c-func docutils literal notranslate">Py_RunMain()</code></a></li>
</ul>
This PEP also adds <code class="docutils literal notranslate">_PyRuntimeState.preconfig</code> (<a class="reference internal" href="../c-api/init_config.html#c.PyPreConfig" title="PyPreConfig"><code class="xref c c-type docutils literal notranslate">PyPreConfig</code></a> type)
and <code class="docutils literal notranslate">PyInterpreterState.config</code> (<a class="reference internal" href="../c-api/init_config.html#c.PyConfig" title="PyConfig"><code class="xref c c-type docutils literal notranslate">PyConfig</code></a> type) fields to these
internal structures. <code class="docutils literal notranslate">PyInterpreterState.config</code> becomes the new
reference configuration, replacing global configuration variables and
other private variables.
See <a class="reference internal" href="../c-api/init_config.html#init-config">Python Initialization Configuration</a> for the
documentation.
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0587">PEP 587</a> for a full description.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36763">bpo-36763</a>.)
</section>
<section id="pep-590-vectorcall-a-fast-calling-protocol-for-cpython">
<h3>PEP 590: Vectorcall: a fast calling protocol for CPython<a class="headerlink" href="#pep-590-vectorcall-a-fast-calling-protocol-for-cpython" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../c-api/call.html#vectorcall">The Vectorcall Protocol</a> is added to the Python/C API.
It is meant to formalize existing optimizations which were already done
for various classes.
Any <a class="reference internal" href="../c-api/typeobj.html#static-types">static type</a> implementing a callable can use this
protocol.
This is currently provisional.
The aim is to make it fully public in Python 3.9.
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0590">PEP 590</a> for a full description.
(Contributed by Jeroen Demeyer, Mark Shannon and Petr Viktorin in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36974">bpo-36974</a>.)
</section>
<section id="pickle-protocol-5-with-out-of-band-data-buffers">
<h3>Pickle protocol 5 with out-of-band data buffers<a class="headerlink" href="#pickle-protocol-5-with-out-of-band-data-buffers" title="Permalink to this headline">¶</a></h3>
When <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> is used to transfer large data between Python processes
in order to take advantage of multi-core or multi-machine processing,
it is important to optimize the transfer by reducing memory copies, and
possibly by applying custom techniques such as data-dependent compression.
The <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> protocol 5 introduces support for out-of-band buffers
where <a class="pep reference external" href="https://www.python.org/dev/peps/pep-3118">PEP 3118</a>-compatible data can be transmitted separately from the
main pickle stream, at the discretion of the communication layer.
See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0574">PEP 574</a> for a full description.
(Contributed by Antoine Pitrou in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36785">bpo-36785</a>.)
</section>
</section>
<section id="other-language-changes">
<h2>Other Language Changes<a class="headerlink" href="#other-language-changes" title="Permalink to this headline">¶</a></h2>
<ul>
<li>A <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> statement was illegal in the <a class="reference internal" href="../reference/compound_stmts.html#finally"><code class="xref std std-keyword docutils literal notranslate">finally</code></a> clause
due to a problem with the implementation. In Python 3.8 this restriction
was lifted.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32489">bpo-32489</a>.)</li>
<li>The <a class="reference internal" href="../library/functions.html#bool" title="bool"><code class="xref py py-class docutils literal notranslate">bool</code></a>, <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>, and <a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate">fractions.Fraction</code></a> types
now have an <a class="reference internal" href="../library/stdtypes.html#int.as_integer_ratio" title="int.as_integer_ratio"><code class="xref py py-meth docutils literal notranslate">as_integer_ratio()</code></a> method like that found in
<a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a> and <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate">decimal.Decimal</code></a>. This minor API extension
makes it possible to write <code class="docutils literal notranslate">numerator, denominator =
x.as_integer_ratio()</code> and have it work across multiple numeric types.
(Contributed by Lisa Roach in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33073">bpo-33073</a> and Raymond Hettinger in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37819">bpo-37819</a>.)</li>
<li>Constructors of <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>, <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a> and <a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-class docutils literal notranslate">complex</code></a> will now
use the <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate">__index__()</code></a> special method, if available and the
corresponding method <a class="reference internal" href="../reference/datamodel.html#object.__int__" title="object.__int__"><code class="xref py py-meth docutils literal notranslate">__int__()</code></a>, <a class="reference internal" href="../reference/datamodel.html#object.__float__" title="object.__float__"><code class="xref py py-meth docutils literal notranslate">__float__()</code></a>
or <a class="reference internal" href="../reference/datamodel.html#object.__complex__" title="object.__complex__"><code class="xref py py-meth docutils literal notranslate">__complex__()</code></a> is not available.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=20092">bpo-20092</a>.)</li>
<li>Added support of <code class="docutils literal notranslate">\N{name}</code> escapes in <a class="reference internal" href="../library/re.html#module-re" title="re: Regular expression operations."><code class="xref py py-mod docutils literal notranslate">regular expressions</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; notice = &#39;Copyright © 2019&#39;
&gt;&gt;&gt; copyright_year_pattern = re.compile(r&#39;\N{copyright sign}\s*(\d{4})&#39;)
&gt;&gt;&gt; int(copyright_year_pattern.search(notice).group(1))
2019
</pre></div>
</div>
(Contributed by Jonathan Eunice and Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=30688">bpo-30688</a>.)
</li>
<li>Dict and dictviews are now iterable in reversed insertion order using
<a class="reference internal" href="../library/functions.html#reversed" title="reversed"><code class="xref py py-func docutils literal notranslate">reversed()</code></a>. (Contributed by Rémi Lapeyre in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33462">bpo-33462</a>.)</li>
<li>The syntax allowed for keyword names in function calls was further
restricted. In particular, <code class="docutils literal notranslate">f((keyword)=arg)</code> is no longer allowed. It was
never intended to permit more than a bare name on the left-hand side of a
keyword argument assignment term.
(Contributed by Benjamin Peterson in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34641">bpo-34641</a>.)</li>
<li>Generalized iterable unpacking in <a class="reference internal" href="../reference/simple_stmts.html#yield"><code class="xref std std-keyword docutils literal notranslate">yield</code></a> and
<a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a> statements no longer requires enclosing parentheses.
This brings the yield and return syntax into better agreement with
normal assignment syntax:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; def parse(family):
 lastname, *members = family.split()
 return lastname.upper(), *members

&gt;&gt;&gt; parse(&#39;simpsons homer marge bart lisa maggie&#39;)
(&#39;SIMPSONS&#39;, &#39;homer&#39;, &#39;marge&#39;, &#39;bart&#39;, &#39;lisa&#39;, &#39;maggie&#39;)
</pre></div>
</div>
(Contributed by David Cuthbert and Jordan Chapman in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32117">bpo-32117</a>.)
</li>
<li>When a comma is missed in code such as <code class="docutils literal notranslate">[(10, 20) (30, 40)]</code>, the
compiler displays a <a class="reference internal" href="../library/exceptions.html#SyntaxWarning" title="SyntaxWarning"><code class="xref py py-exc docutils literal notranslate">SyntaxWarning</code></a> with a helpful suggestion.
This improves on just having a <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate">TypeError</code></a> indicating that the
first tuple was not callable. (Contributed by Serhiy Storchaka in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=15248">bpo-15248</a>.)</li>
<li>Arithmetic operations between subclasses of <a class="reference internal" href="../library/datetime.html#datetime.date" title="datetime.date"><code class="xref py py-class docutils literal notranslate">datetime.date</code></a> or
<a class="reference internal" href="../library/datetime.html#datetime.datetime" title="datetime.datetime"><code class="xref py py-class docutils literal notranslate">datetime.datetime</code></a> and <a class="reference internal" href="../library/datetime.html#datetime.timedelta" title="datetime.timedelta"><code class="xref py py-class docutils literal notranslate">datetime.timedelta</code></a> objects now return
an instance of the subclass, rather than the base class. This also affects
the return type of operations whose implementation (directly or indirectly)
uses <a class="reference internal" href="../library/datetime.html#datetime.timedelta" title="datetime.timedelta"><code class="xref py py-class docutils literal notranslate">datetime.timedelta</code></a> arithmetic, such as
<a class="reference internal" href="../library/datetime.html#datetime.datetime.astimezone" title="datetime.datetime.astimezone"><code class="xref py py-meth docutils literal notranslate">astimezone()</code></a>.
(Contributed by Paul Ganssle in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32417">bpo-32417</a>.)</li>
<li>When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the
resulting <a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate">KeyboardInterrupt</code></a> exception is not caught, the Python process
now exits via a SIGINT signal or with the correct exit code such that the
calling process can detect that it died due to a Ctrl-C. Shells on POSIX
and Windows use this to properly terminate scripts in interactive sessions.
(Contributed by Google via Gregory P. Smith in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1054041">bpo-1054041</a>.)</li>
<li>Some advanced styles of programming require updating the
<a class="reference internal" href="../library/types.html#types.CodeType" title="types.CodeType"><code class="xref py py-class docutils literal notranslate">types.CodeType</code></a> object for an existing function. Since code
objects are immutable, a new code object needs to be created, one
that is modeled on the existing code object. With 19 parameters,
this was somewhat tedious. Now, the new <code class="docutils literal notranslate">replace()</code> method makes
it possible to create a clone with a few altered parameters.
Here’s an example that alters the <a class="reference internal" href="../library/statistics.html#statistics.mean" title="statistics.mean"><code class="xref py py-func docutils literal notranslate">statistics.mean()</code></a> function to
prevent the data parameter from being used as a keyword argument:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from statistics import mean
&gt;&gt;&gt; mean(data=[10, 20, 90])
40
&gt;&gt;&gt; mean.__code__ = mean.__code__.replace(co_posonlyargcount=1)
&gt;&gt;&gt; mean(data=[10, 20, 90])
Traceback (most recent call last):
 ...
TypeError: mean() got some positional-only arguments passed as keyword arguments: &#39;data&#39;
</pre></div>
</div>
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37032">bpo-37032</a>.)
</li>
<li>For integers, the three-argument form of the <a class="reference internal" href="../library/functions.html#pow" title="pow"><code class="xref py py-func docutils literal notranslate">pow()</code></a> function now
permits the exponent to be negative in the case where the base is
relatively prime to the modulus. It then computes a modular inverse to
the base when the exponent is <code class="docutils literal notranslate">-1</code>, and a suitable power of that
inverse for other negative exponents. For example, to compute the
<a class="reference external" href="https://en.wikipedia.org/wiki/Modular_multiplicative_inverse">modular multiplicative inverse</a> of 38
modulo 137, write:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; pow(38, -1, 137)
119
&gt;&gt;&gt; 119 * 38 % 137
1
</pre></div>
</div>
Modular inverses arise in the solution of <a class="reference external" href="https://en.wikipedia.org/wiki/Diophantine_equation">linear Diophantine
equations</a>.
For example, to find integer solutions for <code class="docutils literal notranslate">4258𝑥 + 147𝑦 = 369</code>,
first rewrite as <code class="docutils literal notranslate">4258𝑥 ≡ 369 (mod 147)</code> then solve:
<div class="doctest highlight-default notranslate"><div class="highlight"><pre>&gt;&gt;&gt; x = 369 * pow(4258, -1, 147) % 147
&gt;&gt;&gt; y = (4258 * x - 369) // -147
&gt;&gt;&gt; 4258 * x + 147 * y
369
</pre></div>
</div>
(Contributed by Mark Dickinson in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36027">bpo-36027</a>.)
</li>
<li>Dict comprehensions have been synced-up with dict literals so that the
key is computed first and the value second:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; # Dict comprehension
&gt;&gt;&gt; cast = {input(&#39;role? &#39;): input(&#39;actor? &#39;) for i in range(2)}
role? King Arthur
actor? Chapman
role? Black Knight
actor? Cleese

&gt;&gt;&gt; # Dict literal
&gt;&gt;&gt; cast = {input(&#39;role? &#39;): input(&#39;actor? &#39;)}
role? Sir Robin
actor? Eric Idle
</pre></div>
</div>
The guaranteed execution order is helpful with assignment expressions
because variables assigned in the key expression will be available in
the value expression:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; names = [&#39;Martin von Löwis&#39;, &#39;Łukasz Langa&#39;, &#39;Walter Dörwald&#39;]
&gt;&gt;&gt; {(n := normalize(&#39;NFC&#39;, name)).casefold() : n for name in names}
{&#39;martin von löwis&#39;: &#39;Martin von Löwis&#39;,
 &#39;łukasz langa&#39;: &#39;Łukasz Langa&#39;,
 &#39;walter dörwald&#39;: &#39;Walter Dörwald&#39;}
</pre></div>
</div>
(Contributed by Jörn Heissler in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35224">bpo-35224</a>.)
</li>
<li>The <a class="reference internal" href="../library/pickle.html#object.__reduce__" title="object.__reduce__"><code class="xref py py-meth docutils literal notranslate">object.__reduce__()</code></a> method can now return a tuple from two to
six elements long. Formerly, five was the limit. The new, optional sixth
element is a callable with a <code class="docutils literal notranslate">(obj, state)</code> signature. This allows the
direct control over the state-updating behavior of a specific object. If
not None, this callable will have priority over the object’s
<code class="xref py py-meth docutils literal notranslate">__setstate__()</code> method.
(Contributed by Pierre Glaser and Olivier Grisel in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35900">bpo-35900</a>.)</li>
</ul>
</section>
<section id="new-modules">
<h2>New Modules<a class="headerlink" href="#new-modules" title="Permalink to this headline">¶</a></h2>
<ul>
<li>The new <a class="reference internal" href="../library/importlib.metadata.html#module-importlib.metadata" title="importlib.metadata: The implementation of the importlib metadata."><code class="xref py py-mod docutils literal notranslate">importlib.metadata</code></a> module provides (provisional) support for
reading metadata from third-party packages. For example, it can extract an
installed package’s version number, list of entry points, and more:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; # Note following example requires that the popular &quot;requests&quot;
&gt;&gt;&gt; # package has been installed.
&gt;&gt;&gt;
&gt;&gt;&gt; from importlib.metadata import version, requires, files
&gt;&gt;&gt; version(&#39;requests&#39;)
&#39;2.22.0&#39;
&gt;&gt;&gt; list(requires(&#39;requests&#39;))
[&#39;chardet (&lt;3.1.0,&gt;=3.0.2)&#39;]
&gt;&gt;&gt; list(files(&#39;requests&#39;))[:5]
[PackagePath(&#39;requests-2.22.0.dist-info/INSTALLER&#39;),
 PackagePath(&#39;requests-2.22.0.dist-info/LICENSE&#39;),
 PackagePath(&#39;requests-2.22.0.dist-info/METADATA&#39;),
 PackagePath(&#39;requests-2.22.0.dist-info/RECORD&#39;),
 PackagePath(&#39;requests-2.22.0.dist-info/WHEEL&#39;)]
</pre></div>
</div>
(Contributed by Barry Warsaw and Jason R. Coombs in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34632">bpo-34632</a>.)
</li>
</ul>
</section>
<section id="improved-modules">
<h2>Improved Modules<a class="headerlink" href="#improved-modules" title="Permalink to this headline">¶</a></h2>
<section id="ast">
<h3>ast<a class="headerlink" href="#ast" title="Permalink to this headline">¶</a></h3>
AST nodes now have <code class="docutils literal notranslate">end_lineno</code> and <code class="docutils literal notranslate">end_col_offset</code> attributes,
which give the precise location of the end of the node. (This only
applies to nodes that have <code class="docutils literal notranslate">lineno</code> and <code class="docutils literal notranslate">col_offset</code> attributes.)
New function <a class="reference internal" href="../library/ast.html#ast.get_source_segment" title="ast.get_source_segment"><code class="xref py py-func docutils literal notranslate">ast.get_source_segment()</code></a> returns the source code
for a specific AST node.
(Contributed by Ivan Levkivskyi in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33416">bpo-33416</a>.)
The <a class="reference internal" href="../library/ast.html#ast.parse" title="ast.parse"><code class="xref py py-func docutils literal notranslate">ast.parse()</code></a> function has some new flags:
<ul class="simple">
<li><code class="docutils literal notranslate">type_comments=True</code> causes it to return the text of <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484">PEP 484</a> and
<a class="pep reference external" href="https://www.python.org/dev/peps/pep-0526">PEP 526</a> type comments associated with certain AST nodes;</li>
<li><code class="docutils literal notranslate">mode='func_type'</code> can be used to parse <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0484">PEP 484</a> “signature type
comments” (returned for function definition AST nodes);</li>
<li><code class="docutils literal notranslate">feature_version=(3, N)</code> allows specifying an earlier Python 3
version. For example, <code class="docutils literal notranslate">feature_version=(3, 4)</code> will treat
<a class="reference internal" href="../reference/compound_stmts.html#async"><code class="xref std std-keyword docutils literal notranslate">async</code></a> and <a class="reference internal" href="../reference/expressions.html#await"><code class="xref std std-keyword docutils literal notranslate">await</code></a> as non-reserved words.</li>
</ul>
(Contributed by Guido van Rossum in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35766">bpo-35766</a>.)
</section>
<section id="asyncio">
<h3>asyncio<a class="headerlink" href="#asyncio" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/asyncio-task.html#asyncio.run" title="asyncio.run"><code class="xref py py-func docutils literal notranslate">asyncio.run()</code></a> has graduated from the provisional to stable API. This
function can be used to execute a <a class="reference internal" href="../glossary.html#term-coroutine">coroutine</a> and return the result while
automatically managing the event loop. For example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import asyncio

async def main():
 await asyncio.sleep(0)
 return 42

asyncio.run(main())
</pre></div>
</div>
This is roughly equivalent to:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import asyncio

loop = asyncio.new_event_loop()
asyncio.set_event_loop(loop)
try:
 loop.run_until_complete(main())
finally:
 asyncio.set_event_loop(None)
 loop.close()
</pre></div>
</div>
The actual implementation is significantly more complex. Thus,
<a class="reference internal" href="../library/asyncio-task.html#asyncio.run" title="asyncio.run"><code class="xref py py-func docutils literal notranslate">asyncio.run()</code></a> should be the preferred way of running asyncio programs.
(Contributed by Yury Selivanov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32314">bpo-32314</a>.)
Running <code class="docutils literal notranslate">python -m asyncio</code> launches a natively async REPL. This allows rapid
experimentation with code that has a top-level <a class="reference internal" href="../reference/expressions.html#await"><code class="xref std std-keyword docutils literal notranslate">await</code></a>. There is no
longer a need to directly call <code class="docutils literal notranslate">asyncio.run()</code> which would spawn a new event
loop on every invocation:
<div class="highlight-none notranslate"><div class="highlight"><pre>$ python -m asyncio
asyncio REPL 3.8.0
Use &quot;await&quot; directly instead of &quot;asyncio.run()&quot;.
Type &quot;help&quot;, &quot;copyright&quot;, &quot;credits&quot; or &quot;license&quot; for more information.
&gt;&gt;&gt; import asyncio
&gt;&gt;&gt; await asyncio.sleep(10, result=&#39;hello&#39;)
hello
</pre></div>
</div>
(Contributed by Yury Selivanov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37028">bpo-37028</a>.)
The exception <a class="reference internal" href="../library/asyncio-exceptions.html#asyncio.CancelledError" title="asyncio.CancelledError"><code class="xref py py-class docutils literal notranslate">asyncio.CancelledError</code></a> now inherits from
<a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-class docutils literal notranslate">BaseException</code></a> rather than <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-class docutils literal notranslate">Exception</code></a> and no longer inherits
from <a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.CancelledError" title="concurrent.futures.CancelledError"><code class="xref py py-class docutils literal notranslate">concurrent.futures.CancelledError</code></a>.
(Contributed by Yury Selivanov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32528">bpo-32528</a>.)
On Windows, the default event loop is now <a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.ProactorEventLoop" title="asyncio.ProactorEventLoop"><code class="xref py py-class docutils literal notranslate">ProactorEventLoop</code></a>.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34687">bpo-34687</a>.)
<a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.ProactorEventLoop" title="asyncio.ProactorEventLoop"><code class="xref py py-class docutils literal notranslate">ProactorEventLoop</code></a> now also supports UDP.
(Contributed by Adam Meily and Andrew Svetlov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=29883">bpo-29883</a>.)
<a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.ProactorEventLoop" title="asyncio.ProactorEventLoop"><code class="xref py py-class docutils literal notranslate">ProactorEventLoop</code></a> can now be interrupted by
<a class="reference internal" href="../library/exceptions.html#KeyboardInterrupt" title="KeyboardInterrupt"><code class="xref py py-exc docutils literal notranslate">KeyboardInterrupt</code></a> (“CTRL+C”).
(Contributed by Vladimir Matveev in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=23057">bpo-23057</a>.)
Added <a class="reference internal" href="../library/asyncio-task.html#asyncio.Task.get_coro" title="asyncio.Task.get_coro"><code class="xref py py-meth docutils literal notranslate">asyncio.Task.get_coro()</code></a> for getting the wrapped coroutine
within an <a class="reference internal" href="../library/asyncio-task.html#asyncio.Task" title="asyncio.Task"><code class="xref py py-class docutils literal notranslate">asyncio.Task</code></a>.
(Contributed by Alex Grönholm in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36999">bpo-36999</a>.)
Asyncio tasks can now be named, either by passing the <code class="docutils literal notranslate">name</code> keyword
argument to <a class="reference internal" href="../library/asyncio-task.html#asyncio.create_task" title="asyncio.create_task"><code class="xref py py-func docutils literal notranslate">asyncio.create_task()</code></a> or
the <a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.loop.create_task" title="asyncio.loop.create_task"><code class="xref py py-meth docutils literal notranslate">create_task()</code></a> event loop method, or by
calling the <a class="reference internal" href="../library/asyncio-task.html#asyncio.Task.set_name" title="asyncio.Task.set_name"><code class="xref py py-meth docutils literal notranslate">set_name()</code></a> method on the task object. The
task name is visible in the <code class="docutils literal notranslate">repr()</code> output of <a class="reference internal" href="../library/asyncio-task.html#asyncio.Task" title="asyncio.Task"><code class="xref py py-class docutils literal notranslate">asyncio.Task</code></a> and
can also be retrieved using the <a class="reference internal" href="../library/asyncio-task.html#asyncio.Task.get_name" title="asyncio.Task.get_name"><code class="xref py py-meth docutils literal notranslate">get_name()</code></a> method.
(Contributed by Alex Grönholm in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34270">bpo-34270</a>.)
Added support for
<a class="reference external" href="https://en.wikipedia.org/wiki/Happy_Eyeballs">Happy Eyeballs</a> to
<a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.loop.create_connection" title="asyncio.loop.create_connection"><code class="xref py py-func docutils literal notranslate">asyncio.loop.create_connection()</code></a>. To specify the behavior, two new
parameters have been added: happy_eyeballs_delay and interleave. The Happy
Eyeballs algorithm improves responsiveness in applications that support IPv4
and IPv6 by attempting to simultaneously connect using both.
(Contributed by twisteroid ambassador in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33530">bpo-33530</a>.)
</section>
<section id="builtins">
<h3>builtins<a class="headerlink" href="#builtins" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate">compile()</code></a> built-in has been improved to accept the
<code class="docutils literal notranslate">ast.PyCF_ALLOW_TOP_LEVEL_AWAIT</code> flag. With this new flag passed,
<a class="reference internal" href="../library/functions.html#compile" title="compile"><code class="xref py py-func docutils literal notranslate">compile()</code></a> will allow top-level <code class="docutils literal notranslate">await</code>, <code class="docutils literal notranslate">async for</code> and <code class="docutils literal notranslate">async with</code>
constructs that are usually considered invalid syntax. Asynchronous code object
marked with the <code class="docutils literal notranslate">CO_COROUTINE</code> flag may then be returned.
(Contributed by Matthias Bussonnier in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34616">bpo-34616</a>)
</section>
<section id="collections">
<h3>collections<a class="headerlink" href="#collections" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/collections.html#collections.somenamedtuple._asdict" title="collections.somenamedtuple._asdict"><code class="xref py py-meth docutils literal notranslate">_asdict()</code></a> method for
<a class="reference internal" href="../library/collections.html#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate">collections.namedtuple()</code></a> now returns a <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> instead of a
<a class="reference internal" href="../library/collections.html#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">collections.OrderedDict</code></a>. This works because regular dicts have
guaranteed ordering since Python 3.7. If the extra features of
<code class="xref py py-class docutils literal notranslate">OrderedDict</code> are required, the suggested remediation is to cast the
result to the desired type: <code class="docutils literal notranslate">OrderedDict(nt._asdict())</code>.
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35864">bpo-35864</a>.)
</section>
<section id="cprofile">
<h3>cProfile<a class="headerlink" href="#cprofile" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/profile.html#profile.Profile" title="profile.Profile"><code class="xref py py-class docutils literal notranslate">cProfile.Profile</code></a> class can now be used as a context manager.
Profile a block of code by running:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import cProfile

with cProfile.Profile() as profiler:
 # code to be profiled
 ...
</pre></div>
</div>
(Contributed by Scott Sanderson in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=29235">bpo-29235</a>.)
</section>
<section id="csv">
<h3>csv<a class="headerlink" href="#csv" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/csv.html#csv.DictReader" title="csv.DictReader"><code class="xref py py-class docutils literal notranslate">csv.DictReader</code></a> now returns instances of <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> instead of
a <a class="reference internal" href="../library/collections.html#collections.OrderedDict" title="collections.OrderedDict"><code class="xref py py-class docutils literal notranslate">collections.OrderedDict</code></a>. The tool is now faster and uses less
memory while still preserving the field order.
(Contributed by Michael Selik in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34003">bpo-34003</a>.)
</section>
<section id="curses">
<h3>curses<a class="headerlink" href="#curses" title="Permalink to this headline">¶</a></h3>
Added a new variable holding structured version information for the
underlying ncurses library: <a class="reference internal" href="../library/curses.html#curses.ncurses_version" title="curses.ncurses_version"><code class="xref py py-data docutils literal notranslate">ncurses_version</code></a>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=31680">bpo-31680</a>.)
</section>
<section id="ctypes">
<h3>ctypes<a class="headerlink" href="#ctypes" title="Permalink to this headline">¶</a></h3>
On Windows, <a class="reference internal" href="../library/ctypes.html#ctypes.CDLL" title="ctypes.CDLL"><code class="xref py py-class docutils literal notranslate">CDLL</code></a> and subclasses now accept a winmode parameter
to specify flags for the underlying <code class="docutils literal notranslate">LoadLibraryEx</code> call. The default flags are
set to only load DLL dependencies from trusted locations, including the path
where the DLL is stored (if a full or partial path is used to load the initial
DLL) and paths added by <a class="reference internal" href="../library/os.html#os.add_dll_directory" title="os.add_dll_directory"><code class="xref py py-func docutils literal notranslate">add_dll_directory()</code></a>.
(Contributed by Steve Dower in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36085">bpo-36085</a>.)
</section>
<section id="datetime">
<h3>datetime<a class="headerlink" href="#datetime" title="Permalink to this headline">¶</a></h3>
Added new alternate constructors <a class="reference internal" href="../library/datetime.html#datetime.date.fromisocalendar" title="datetime.date.fromisocalendar"><code class="xref py py-meth docutils literal notranslate">datetime.date.fromisocalendar()</code></a> and
<a class="reference internal" href="../library/datetime.html#datetime.datetime.fromisocalendar" title="datetime.datetime.fromisocalendar"><code class="xref py py-meth docutils literal notranslate">datetime.datetime.fromisocalendar()</code></a>, which construct <code class="xref py py-class docutils literal notranslate">date</code> and
<a class="reference internal" href="../library/datetime.html#module-datetime" title="datetime: Basic date and time types."><code class="xref py py-class docutils literal notranslate">datetime</code></a> objects respectively from ISO year, week number, and weekday;
these are the inverse of each class’s <code class="docutils literal notranslate">isocalendar</code> method.
(Contributed by Paul Ganssle in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36004">bpo-36004</a>.)
</section>
<section id="functools">
<h3>functools<a class="headerlink" href="#functools" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/functools.html#functools.lru_cache" title="functools.lru_cache"><code class="xref py py-func docutils literal notranslate">functools.lru_cache()</code></a> can now be used as a straight decorator rather
than as a function returning a decorator. So both of these are now supported:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>@lru_cache
def f(x):
 ...

@lru_cache(maxsize=256)
def f(x):
 ...
</pre></div>
</div>
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36772">bpo-36772</a>.)
Added a new <a class="reference internal" href="../library/functools.html#functools.cached_property" title="functools.cached_property"><code class="xref py py-func docutils literal notranslate">functools.cached_property()</code></a> decorator, for computed properties
cached for the life of the instance.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import functools
import statistics

class Dataset:
 def __init__(self, sequence_of_numbers):
 self.data = sequence_of_numbers

@functools.cached_property
 def variance(self):
 return statistics.variance(self.data)
</pre></div>
</div>
(Contributed by Carl Meyer in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=21145">bpo-21145</a>)
Added a new <a class="reference internal" href="../library/functools.html#functools.singledispatchmethod" title="functools.singledispatchmethod"><code class="xref py py-func docutils literal notranslate">functools.singledispatchmethod()</code></a> decorator that converts
methods into <a class="reference internal" href="../glossary.html#term-generic-function">generic functions</a> using
<a class="reference internal" href="../glossary.html#term-single-dispatch">single dispatch</a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>from functools import singledispatchmethod
from contextlib import suppress

class TaskManager:

def __init__(self, tasks):
 self.tasks = list(tasks)

@singledispatchmethod
 def discard(self, value):
 with suppress(ValueError):
 self.tasks.remove(value)

@discard.register(list)
 def _(self, tasks):
 targets = set(tasks)
 self.tasks = [x for x in self.tasks if x not in targets]
</pre></div>
</div>
(Contributed by Ethan Smith in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32380">bpo-32380</a>)
</section>
<section id="gc">
<h3>gc<a class="headerlink" href="#gc" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/gc.html#gc.get_objects" title="gc.get_objects"><code class="xref py py-func docutils literal notranslate">get_objects()</code></a> can now receive an optional generation parameter
indicating a generation to get objects from.
(Contributed by Pablo Galindo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36016">bpo-36016</a>.)
</section>
<section id="gettext">
<h3>gettext<a class="headerlink" href="#gettext" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/gettext.html#gettext.pgettext" title="gettext.pgettext"><code class="xref py py-func docutils literal notranslate">pgettext()</code></a> and its variants.
(Contributed by Franz Glasner, Éric Araujo, and Cheryl Sabella in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=2504">bpo-2504</a>.)
</section>
<section id="gzip">
<h3>gzip<a class="headerlink" href="#gzip" title="Permalink to this headline">¶</a></h3>
Added the mtime parameter to <a class="reference internal" href="../library/gzip.html#gzip.compress" title="gzip.compress"><code class="xref py py-func docutils literal notranslate">gzip.compress()</code></a> for reproducible output.
(Contributed by Guo Ci Teo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34898">bpo-34898</a>.)
A <a class="reference internal" href="../library/gzip.html#gzip.BadGzipFile" title="gzip.BadGzipFile"><code class="xref py py-exc docutils literal notranslate">BadGzipFile</code></a> exception is now raised instead of <a class="reference internal" href="../library/exceptions.html#OSError" title="OSError"><code class="xref py py-exc docutils literal notranslate">OSError</code></a>
for certain types of invalid or corrupt gzip files.
(Contributed by Filip Gruszczyński, Michele Orrù, and Zackery Spytz in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=6584">bpo-6584</a>.)
</section>
<section id="idle-and-idlelib">
<h3>IDLE and idlelib<a class="headerlink" href="#idle-and-idlelib" title="Permalink to this headline">¶</a></h3>
Output over N lines (50 by default) is squeezed down to a button.
N can be changed in the PyShell section of the General page of the
Settings dialog. Fewer, but possibly extra long, lines can be squeezed by
right clicking on the output. Squeezed output can be expanded in place
by double-clicking the button or into the clipboard or a separate window
by right-clicking the button. (Contributed by Tal Einat in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1529353">bpo-1529353</a>.)
Add “Run Customized” to the Run menu to run a module with customized
settings. Any command line arguments entered are added to sys.argv.
They also re-appear in the box for the next customized run. One can also
suppress the normal Shell main module restart. (Contributed by Cheryl
Sabella, Terry Jan Reedy, and others in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=5680">bpo-5680</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37627">bpo-37627</a>.)
Added optional line numbers for IDLE editor windows. Windows
open without line numbers unless set otherwise in the General
tab of the configuration dialog. Line numbers for an existing
window are shown and hidden in the Options menu.
(Contributed by Tal Einat and Saimadhav Heblikar in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=17535">bpo-17535</a>.)
OS native encoding is now used for converting between Python strings and Tcl
objects. This allows IDLE to work with emoji and other non-BMP characters.
These characters can be displayed or copied and pasted to or from the
clipboard. Converting strings from Tcl to Python and back now never fails.
(Many people worked on this for eight years but the problem was finally
solved by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=13153">bpo-13153</a>.)
New in 3.8.1:
Add option to toggle cursor blink off. (Contributed by Zackery Spytz
in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=4603">bpo-4603</a>.)
Escape key now closes IDLE completion windows. (Contributed by Johnny
Najera in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38944">bpo-38944</a>.)
The changes above have been backported to 3.7 maintenance releases.
Add keywords to module name completion list. (Contributed by Terry J.
Reedy in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37765">bpo-37765</a>.)
</section>
<section id="inspect">
<h3>inspect<a class="headerlink" href="#inspect" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/inspect.html#inspect.getdoc" title="inspect.getdoc"><code class="xref py py-func docutils literal notranslate">inspect.getdoc()</code></a> function can now find docstrings for <code class="docutils literal notranslate">__slots__</code>
if that attribute is a <a class="reference internal" href="../library/stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate">dict</code></a> where the values are docstrings.
This provides documentation options similar to what we already have
for <a class="reference internal" href="../library/functions.html#property" title="property"><code class="xref py py-func docutils literal notranslate">property()</code></a>, <a class="reference internal" href="../library/functions.html#classmethod" title="classmethod"><code class="xref py py-func docutils literal notranslate">classmethod()</code></a>, and <a class="reference internal" href="../library/functions.html#staticmethod" title="staticmethod"><code class="xref py py-func docutils literal notranslate">staticmethod()</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class AudioClip:
 __slots__ = {&#39;bit_rate&#39;: &#39;expressed in kilohertz to one decimal place&#39;,
 &#39;duration&#39;: &#39;in seconds, rounded up to an integer&#39;}
 def __init__(self, bit_rate, duration):
 self.bit_rate = round(bit_rate / 1000.0, 1)
 self.duration = ceil(duration)
</pre></div>
</div>
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36326">bpo-36326</a>.)
</section>
<section id="io">
<h3>io<a class="headerlink" href="#io" title="Permalink to this headline">¶</a></h3>
In development mode (<a class="reference internal" href="../using/cmdline.html#cmdoption-X"><code class="xref std std-option docutils literal notranslate">-X</code></a> <code class="docutils literal notranslate">env</code>) and in <a class="reference internal" href="../using/configure.html#debug-build">debug build</a>, the
<a class="reference internal" href="../library/io.html#io.IOBase" title="io.IOBase"><code class="xref py py-class docutils literal notranslate">io.IOBase</code></a> finalizer now logs the exception if the <code class="docutils literal notranslate">close()</code> method
fails. The exception is ignored silently by default in release build.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=18748">bpo-18748</a>.)
</section>
<section id="itertools">
<h3>itertools<a class="headerlink" href="#itertools" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/itertools.html#itertools.accumulate" title="itertools.accumulate"><code class="xref py py-func docutils literal notranslate">itertools.accumulate()</code></a> function added an option initial keyword
argument to specify an initial value:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from itertools import accumulate
&gt;&gt;&gt; list(accumulate([10, 5, 30, 15], initial=1000))
[1000, 1010, 1015, 1045, 1060]
</pre></div>
</div>
(Contributed by Lisa Roach in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34659">bpo-34659</a>.)
</section>
<section id="json-tool">
<h3>json.tool<a class="headerlink" href="#json-tool" title="Permalink to this headline">¶</a></h3>
Add option <code class="docutils literal notranslate">--json-lines</code> to parse every input line as a separate JSON object.
(Contributed by Weipeng Hong in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=31553">bpo-31553</a>.)
</section>
<section id="logging">
<h3>logging<a class="headerlink" href="#logging" title="Permalink to this headline">¶</a></h3>
Added a force keyword argument to <a class="reference internal" href="../library/logging.html#logging.basicConfig" title="logging.basicConfig"><code class="xref py py-func docutils literal notranslate">logging.basicConfig()</code></a>
When set to true, any existing handlers attached
to the root logger are removed and closed before carrying out the
configuration specified by the other arguments.
This solves a long-standing problem. Once a logger or basicConfig() had
been called, subsequent calls to basicConfig() were silently ignored.
This made it difficult to update, experiment with, or teach the various
logging configuration options using the interactive prompt or a Jupyter
notebook.
(Suggested by Raymond Hettinger, implemented by Dong-hee Na, and
reviewed by Vinay Sajip in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33897">bpo-33897</a>.)
</section>
<section id="math">
<h3>math<a class="headerlink" href="#math" title="Permalink to this headline">¶</a></h3>
Added new function <a class="reference internal" href="../library/math.html#math.dist" title="math.dist"><code class="xref py py-func docutils literal notranslate">math.dist()</code></a> for computing Euclidean distance
between two points. (Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33089">bpo-33089</a>.)
Expanded the <a class="reference internal" href="../library/math.html#math.hypot" title="math.hypot"><code class="xref py py-func docutils literal notranslate">math.hypot()</code></a> function to handle multiple dimensions.
Formerly, it only supported the 2-D case.
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33089">bpo-33089</a>.)
Added new function, <a class="reference internal" href="../library/math.html#math.prod" title="math.prod"><code class="xref py py-func docutils literal notranslate">math.prod()</code></a>, as analogous function to <a class="reference internal" href="../library/functions.html#sum" title="sum"><code class="xref py py-func docutils literal notranslate">sum()</code></a>
that returns the product of a ‘start’ value (default: 1) times an iterable of
numbers:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; prior = 0.8
&gt;&gt;&gt; likelihoods = [0.625, 0.84, 0.30]
&gt;&gt;&gt; math.prod(likelihoods, start=prior)
0.126
</pre></div>
</div>
(Contributed by Pablo Galindo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35606">bpo-35606</a>.)
Added two new combinatoric functions <a class="reference internal" href="../library/math.html#math.perm" title="math.perm"><code class="xref py py-func docutils literal notranslate">math.perm()</code></a> and <a class="reference internal" href="../library/math.html#math.comb" title="math.comb"><code class="xref py py-func docutils literal notranslate">math.comb()</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; math.perm(10, 3) # Permutations of 10 things taken 3 at a time
720
&gt;&gt;&gt; math.comb(10, 3) # Combinations of 10 things taken 3 at a time
120
</pre></div>
</div>
(Contributed by Yash Aggarwal, Keller Fuchs, Serhiy Storchaka, and Raymond
Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37128">bpo-37128</a>, <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37178">bpo-37178</a>, and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35431">bpo-35431</a>.)
Added a new function <a class="reference internal" href="../library/math.html#math.isqrt" title="math.isqrt"><code class="xref py py-func docutils literal notranslate">math.isqrt()</code></a> for computing accurate integer square
roots without conversion to floating point. The new function supports
arbitrarily large integers. It is faster than <code class="docutils literal notranslate">floor(sqrt(n))</code> but slower
than <a class="reference internal" href="../library/math.html#math.sqrt" title="math.sqrt"><code class="xref py py-func docutils literal notranslate">math.sqrt()</code></a>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; r = 650320427
&gt;&gt;&gt; s = r ** 2
&gt;&gt;&gt; isqrt(s - 1) # correct
650320426
&gt;&gt;&gt; floor(sqrt(s - 1)) # incorrect
650320427
</pre></div>
</div>
(Contributed by Mark Dickinson in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36887">bpo-36887</a>.)
The function <a class="reference internal" href="../library/math.html#math.factorial" title="math.factorial"><code class="xref py py-func docutils literal notranslate">math.factorial()</code></a> no longer accepts arguments that are not
int-like. (Contributed by Pablo Galindo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33083">bpo-33083</a>.)
</section>
<section id="mmap">
<h3>mmap<a class="headerlink" href="#mmap" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/mmap.html#mmap.mmap" title="mmap.mmap"><code class="xref py py-class docutils literal notranslate">mmap.mmap</code></a> class now has an <a class="reference internal" href="../library/mmap.html#mmap.mmap.madvise" title="mmap.mmap.madvise"><code class="xref py py-meth docutils literal notranslate">madvise()</code></a> method to
access the <code class="docutils literal notranslate">madvise()</code> system call.
(Contributed by Zackery Spytz in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32941">bpo-32941</a>.)
</section>
<section id="multiprocessing">
<h3>multiprocessing<a class="headerlink" href="#multiprocessing" title="Permalink to this headline">¶</a></h3>
Added new <a class="reference internal" href="../library/multiprocessing.shared_memory.html#module-multiprocessing.shared_memory" title="multiprocessing.shared_memory: Provides shared memory for direct access across processes."><code class="xref py py-mod docutils literal notranslate">multiprocessing.shared_memory</code></a> module.
(Contributed by Davin Potts in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35813">bpo-35813</a>.)
On macOS, the spawn start method is now used by default.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33725">bpo-33725</a>.)
</section>
<section id="os">
<h3>os<a class="headerlink" href="#os" title="Permalink to this headline">¶</a></h3>
Added new function <a class="reference internal" href="../library/os.html#os.add_dll_directory" title="os.add_dll_directory"><code class="xref py py-func docutils literal notranslate">add_dll_directory()</code></a> on Windows for providing
additional search paths for native dependencies when importing extension
modules or loading DLLs using <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a>.
(Contributed by Steve Dower in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36085">bpo-36085</a>.)
A new <a class="reference internal" href="../library/os.html#os.memfd_create" title="os.memfd_create"><code class="xref py py-func docutils literal notranslate">os.memfd_create()</code></a> function was added to wrap the
<code class="docutils literal notranslate">memfd_create()</code> syscall.
(Contributed by Zackery Spytz and Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=26836">bpo-26836</a>.)
On Windows, much of the manual logic for handling reparse points (including
symlinks and directory junctions) has been delegated to the operating system.
Specifically, <a class="reference internal" href="../library/os.html#os.stat" title="os.stat"><code class="xref py py-func docutils literal notranslate">os.stat()</code></a> will now traverse anything supported by the
operating system, while <a class="reference internal" href="../library/os.html#os.lstat" title="os.lstat"><code class="xref py py-func docutils literal notranslate">os.lstat()</code></a> will only open reparse points that
identify as “name surrogates” while others are opened as for <a class="reference internal" href="../library/os.html#os.stat" title="os.stat"><code class="xref py py-func docutils literal notranslate">os.stat()</code></a>.
In all cases, <code class="xref py py-attr docutils literal notranslate">stat_result.st_mode</code> will only have <code class="docutils literal notranslate">S_IFLNK</code> set for
symbolic links and not other kinds of reparse points. To identify other kinds
of reparse point, check the new <code class="xref py py-attr docutils literal notranslate">stat_result.st_reparse_tag</code> attribute.
On Windows, <a class="reference internal" href="../library/os.html#os.readlink" title="os.readlink"><code class="xref py py-func docutils literal notranslate">os.readlink()</code></a> is now able to read directory junctions. Note
that <a class="reference internal" href="../library/os.path.html#os.path.islink" title="os.path.islink"><code class="xref py py-func docutils literal notranslate">islink()</code></a> will return <code class="docutils literal notranslate">False</code> for directory junctions,
and so code that checks <code class="docutils literal notranslate">islink</code> first will continue to treat junctions as
directories, while code that handles errors from <a class="reference internal" href="../library/os.html#os.readlink" title="os.readlink"><code class="xref py py-func docutils literal notranslate">os.readlink()</code></a> may now
treat junctions as links.
(Contributed by Steve Dower in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37834">bpo-37834</a>.)
</section>
<section id="os-path">
<h3>os.path<a class="headerlink" href="#os-path" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/os.path.html#module-os.path" title="os.path: Operations on pathnames."><code class="xref py py-mod docutils literal notranslate">os.path</code></a> functions that return a boolean result like
<a class="reference internal" href="../library/os.path.html#os.path.exists" title="os.path.exists"><code class="xref py py-func docutils literal notranslate">exists()</code></a>, <a class="reference internal" href="../library/os.path.html#os.path.lexists" title="os.path.lexists"><code class="xref py py-func docutils literal notranslate">lexists()</code></a>, <a class="reference internal" href="../library/os.path.html#os.path.isdir" title="os.path.isdir"><code class="xref py py-func docutils literal notranslate">isdir()</code></a>,
<a class="reference internal" href="../library/os.path.html#os.path.isfile" title="os.path.isfile"><code class="xref py py-func docutils literal notranslate">isfile()</code></a>, <a class="reference internal" href="../library/os.path.html#os.path.islink" title="os.path.islink"><code class="xref py py-func docutils literal notranslate">islink()</code></a>, and <a class="reference internal" href="../library/os.path.html#os.path.ismount" title="os.path.ismount"><code class="xref py py-func docutils literal notranslate">ismount()</code></a>
now return <code class="docutils literal notranslate">False</code> instead of raising <a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> or its subclasses
<a class="reference internal" href="../library/exceptions.html#UnicodeEncodeError" title="UnicodeEncodeError"><code class="xref py py-exc docutils literal notranslate">UnicodeEncodeError</code></a> and <a class="reference internal" href="../library/exceptions.html#UnicodeDecodeError" title="UnicodeDecodeError"><code class="xref py py-exc docutils literal notranslate">UnicodeDecodeError</code></a> for paths that contain
characters or bytes unrepresentable at the OS level.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33721">bpo-33721</a>.)
<a class="reference internal" href="../library/os.path.html#os.path.expanduser" title="os.path.expanduser"><code class="xref py py-func docutils literal notranslate">expanduser()</code></a> on Windows now prefers the <code class="xref std std-envvar docutils literal notranslate">USERPROFILE</code>
environment variable and does not use <code class="xref std std-envvar docutils literal notranslate">HOME</code>, which is not normally set
for regular user accounts.
(Contributed by Anthony Sottile in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36264">bpo-36264</a>.)
<a class="reference internal" href="../library/os.path.html#os.path.isdir" title="os.path.isdir"><code class="xref py py-func docutils literal notranslate">isdir()</code></a> on Windows no longer returns <code class="docutils literal notranslate">True</code> for a link to a
non-existent directory.
<a class="reference internal" href="../library/os.path.html#os.path.realpath" title="os.path.realpath"><code class="xref py py-func docutils literal notranslate">realpath()</code></a> on Windows now resolves reparse points, including
symlinks and directory junctions.
(Contributed by Steve Dower in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37834">bpo-37834</a>.)
</section>
<section id="pathlib">
<h3>pathlib<a class="headerlink" href="#pathlib" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/pathlib.html#pathlib.Path" title="pathlib.Path"><code class="xref py py-mod docutils literal notranslate">pathlib.Path</code></a> methods that return a boolean result like
<a class="reference internal" href="../library/pathlib.html#pathlib.Path.exists" title="pathlib.Path.exists"><code class="xref py py-meth docutils literal notranslate">exists()</code></a>, <a class="reference internal" href="../library/pathlib.html#pathlib.Path.is_dir" title="pathlib.Path.is_dir"><code class="xref py py-meth docutils literal notranslate">is_dir()</code></a>,
<a class="reference internal" href="../library/pathlib.html#pathlib.Path.is_file" title="pathlib.Path.is_file"><code class="xref py py-meth docutils literal notranslate">is_file()</code></a>, <a class="reference internal" href="../library/pathlib.html#pathlib.Path.is_mount" title="pathlib.Path.is_mount"><code class="xref py py-meth docutils literal notranslate">is_mount()</code></a>,
<a class="reference internal" href="../library/pathlib.html#pathlib.Path.is_symlink" title="pathlib.Path.is_symlink"><code class="xref py py-meth docutils literal notranslate">is_symlink()</code></a>, <a class="reference internal" href="../library/pathlib.html#pathlib.Path.is_block_device" title="pathlib.Path.is_block_device"><code class="xref py py-meth docutils literal notranslate">is_block_device()</code></a>,
<a class="reference internal" href="../library/pathlib.html#pathlib.Path.is_char_device" title="pathlib.Path.is_char_device"><code class="xref py py-meth docutils literal notranslate">is_char_device()</code></a>, <a class="reference internal" href="../library/pathlib.html#pathlib.Path.is_fifo" title="pathlib.Path.is_fifo"><code class="xref py py-meth docutils literal notranslate">is_fifo()</code></a>,
<a class="reference internal" href="../library/pathlib.html#pathlib.Path.is_socket" title="pathlib.Path.is_socket"><code class="xref py py-meth docutils literal notranslate">is_socket()</code></a> now return <code class="docutils literal notranslate">False</code> instead of raising
<a class="reference internal" href="../library/exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a> or its subclass <a class="reference internal" href="../library/exceptions.html#UnicodeEncodeError" title="UnicodeEncodeError"><code class="xref py py-exc docutils literal notranslate">UnicodeEncodeError</code></a> for paths that
contain characters unrepresentable at the OS level.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33721">bpo-33721</a>.)
Added <a class="reference internal" href="../library/pathlib.html#pathlib.Path.link_to" title="pathlib.Path.link_to"><code class="xref py py-meth docutils literal notranslate">pathlib.Path.link_to()</code></a> which creates a hard link pointing
to a path.
(Contributed by Joannah Nanjekye in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=26978">bpo-26978</a>)
</section>
<section id="pickle">
<h3>pickle<a class="headerlink" href="#pickle" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> extensions subclassing the C-optimized <a class="reference internal" href="../library/pickle.html#pickle.Pickler" title="pickle.Pickler"><code class="xref py py-class docutils literal notranslate">Pickler</code></a>
can now override the pickling logic of functions and classes by defining the
special <a class="reference internal" href="../library/pickle.html#pickle.Pickler.reducer_override" title="pickle.Pickler.reducer_override"><code class="xref py py-meth docutils literal notranslate">reducer_override()</code></a> method.
(Contributed by Pierre Glaser and Olivier Grisel in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35900">bpo-35900</a>.)
</section>
<section id="plistlib">
<h3>plistlib<a class="headerlink" href="#plistlib" title="Permalink to this headline">¶</a></h3>
Added new <a class="reference internal" href="../library/plistlib.html#plistlib.UID" title="plistlib.UID"><code class="xref py py-class docutils literal notranslate">plistlib.UID</code></a> and enabled support for reading and writing
NSKeyedArchiver-encoded binary plists.
(Contributed by Jon Janzen in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=26707">bpo-26707</a>.)
</section>
<section id="pprint">
<h3>pprint<a class="headerlink" href="#pprint" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/pprint.html#module-pprint" title="pprint: Data pretty printer."><code class="xref py py-mod docutils literal notranslate">pprint</code></a> module added a sort_dicts parameter to several functions.
By default, those functions continue to sort dictionaries before rendering or
printing. However, if sort_dicts is set to false, the dictionaries retain
the order that keys were inserted. This can be useful for comparison to JSON
inputs during debugging.
In addition, there is a convenience new function, <a class="reference internal" href="../library/pprint.html#pprint.pp" title="pprint.pp"><code class="xref py py-func docutils literal notranslate">pprint.pp()</code></a> that is
like <a class="reference internal" href="../library/pprint.html#pprint.pprint" title="pprint.pprint"><code class="xref py py-func docutils literal notranslate">pprint.pprint()</code></a> but with sort_dicts defaulting to <code class="docutils literal notranslate">False</code>:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; from pprint import pprint, pp
&gt;&gt;&gt; d = dict(source=&#39;input.txt&#39;, operation=&#39;filter&#39;, destination=&#39;output.txt&#39;)
&gt;&gt;&gt; pp(d, width=40) # Original order
{&#39;source&#39;: &#39;input.txt&#39;,
 &#39;operation&#39;: &#39;filter&#39;,
 &#39;destination&#39;: &#39;output.txt&#39;}
&gt;&gt;&gt; pprint(d, width=40) # Keys sorted alphabetically
{&#39;destination&#39;: &#39;output.txt&#39;,
 &#39;operation&#39;: &#39;filter&#39;,
 &#39;source&#39;: &#39;input.txt&#39;}
</pre></div>
</div>
(Contributed by Rémi Lapeyre in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=30670">bpo-30670</a>.)
</section>
<section id="py-compile">
<h3>py_compile<a class="headerlink" href="#py-compile" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/py_compile.html#py_compile.compile" title="py_compile.compile"><code class="xref py py-func docutils literal notranslate">py_compile.compile()</code></a> now supports silent mode.
(Contributed by Joannah Nanjekye in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=22640">bpo-22640</a>.)
</section>
<section id="shlex">
<h3>shlex<a class="headerlink" href="#shlex" title="Permalink to this headline">¶</a></h3>
The new <a class="reference internal" href="../library/shlex.html#shlex.join" title="shlex.join"><code class="xref py py-func docutils literal notranslate">shlex.join()</code></a> function acts as the inverse of <a class="reference internal" href="../library/shlex.html#shlex.split" title="shlex.split"><code class="xref py py-func docutils literal notranslate">shlex.split()</code></a>.
(Contributed by Bo Bayles in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32102">bpo-32102</a>.)
</section>
<section id="shutil">
<h3>shutil<a class="headerlink" href="#shutil" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/shutil.html#shutil.copytree" title="shutil.copytree"><code class="xref py py-func docutils literal notranslate">shutil.copytree()</code></a> now accepts a new <code class="docutils literal notranslate">dirs_exist_ok</code> keyword argument.
(Contributed by Josh Bronson in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=20849">bpo-20849</a>.)
<a class="reference internal" href="../library/shutil.html#shutil.make_archive" title="shutil.make_archive"><code class="xref py py-func docutils literal notranslate">shutil.make_archive()</code></a> now defaults to the modern pax (POSIX.1-2001)
format for new archives to improve portability and standards conformance,
inherited from the corresponding change to the <a class="reference internal" href="../library/tarfile.html#module-tarfile" title="tarfile: Read and write tar-format archive files."><code class="xref py py-mod docutils literal notranslate">tarfile</code></a> module.
(Contributed by C.A.M. Gerlach in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=30661">bpo-30661</a>.)
<a class="reference internal" href="../library/shutil.html#shutil.rmtree" title="shutil.rmtree"><code class="xref py py-func docutils literal notranslate">shutil.rmtree()</code></a> on Windows now removes directory junctions without
recursively removing their contents first.
(Contributed by Steve Dower in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37834">bpo-37834</a>.)
</section>
<section id="socket">
<h3>socket<a class="headerlink" href="#socket" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/socket.html#socket.create_server" title="socket.create_server"><code class="xref py py-meth docutils literal notranslate">create_server()</code></a> and <a class="reference internal" href="../library/socket.html#socket.has_dualstack_ipv6" title="socket.has_dualstack_ipv6"><code class="xref py py-meth docutils literal notranslate">has_dualstack_ipv6()</code></a>
convenience functions to automate the necessary tasks usually involved when
creating a server socket, including accepting both IPv4 and IPv6 connections
on the same socket. (Contributed by Giampaolo Rodolà in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=17561">bpo-17561</a>.)
The <a class="reference internal" href="../library/socket.html#socket.if_nameindex" title="socket.if_nameindex"><code class="xref py py-func docutils literal notranslate">socket.if_nameindex()</code></a>, <a class="reference internal" href="../library/socket.html#socket.if_nametoindex" title="socket.if_nametoindex"><code class="xref py py-func docutils literal notranslate">socket.if_nametoindex()</code></a>, and
<a class="reference internal" href="../library/socket.html#socket.if_indextoname" title="socket.if_indextoname"><code class="xref py py-func docutils literal notranslate">socket.if_indextoname()</code></a> functions have been implemented on Windows.
(Contributed by Zackery Spytz in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37007">bpo-37007</a>.)
</section>
<section id="ssl">
<h3>ssl<a class="headerlink" href="#ssl" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/ssl.html#ssl.SSLContext.post_handshake_auth" title="ssl.SSLContext.post_handshake_auth"><code class="xref py py-attr docutils literal notranslate">post_handshake_auth</code></a> to enable and
<a class="reference internal" href="../library/ssl.html#ssl.SSLSocket.verify_client_post_handshake" title="ssl.SSLSocket.verify_client_post_handshake"><code class="xref py py-meth docutils literal notranslate">verify_client_post_handshake()</code></a> to initiate TLS 1.3
post-handshake authentication.
(Contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34670">bpo-34670</a>.)
</section>
<section id="statistics">
<h3>statistics<a class="headerlink" href="#statistics" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/statistics.html#statistics.fmean" title="statistics.fmean"><code class="xref py py-func docutils literal notranslate">statistics.fmean()</code></a> as a faster, floating point variant of
<a class="reference internal" href="../library/statistics.html#statistics.mean" title="statistics.mean"><code class="xref py py-func docutils literal notranslate">statistics.mean()</code></a>. (Contributed by Raymond Hettinger and
Steven D’Aprano in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35904">bpo-35904</a>.)
Added <a class="reference internal" href="../library/statistics.html#statistics.geometric_mean" title="statistics.geometric_mean"><code class="xref py py-func docutils literal notranslate">statistics.geometric_mean()</code></a>
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=27181">bpo-27181</a>.)
Added <a class="reference internal" href="../library/statistics.html#statistics.multimode" title="statistics.multimode"><code class="xref py py-func docutils literal notranslate">statistics.multimode()</code></a> that returns a list of the most
common values. (Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35892">bpo-35892</a>.)
Added <a class="reference internal" href="../library/statistics.html#statistics.quantiles" title="statistics.quantiles"><code class="xref py py-func docutils literal notranslate">statistics.quantiles()</code></a> that divides data or a distribution
in to equiprobable intervals (e.g. quartiles, deciles, or percentiles).
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36546">bpo-36546</a>.)
Added <a class="reference internal" href="../library/statistics.html#statistics.NormalDist" title="statistics.NormalDist"><code class="xref py py-class docutils literal notranslate">statistics.NormalDist</code></a>, a tool for creating
and manipulating normal distributions of a random variable.
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36018">bpo-36018</a>.)
<div class="highlight-python3 notranslate"><div class="highlight"><pre>&gt;&gt;&gt; temperature_feb = NormalDist.from_samples([4, 12, -3, 2, 7, 14])
&gt;&gt;&gt; temperature_feb.mean
6.0
&gt;&gt;&gt; temperature_feb.stdev
6.356099432828281

&gt;&gt;&gt; temperature_feb.cdf(3) # Chance of being under 3 degrees
0.3184678262814532
&gt;&gt;&gt; # Relative chance of being 7 degrees versus 10 degrees
&gt;&gt;&gt; temperature_feb.pdf(7) / temperature_feb.pdf(10)
1.2039930378537762

&gt;&gt;&gt; el_niño = NormalDist(4, 2.5)
&gt;&gt;&gt; temperature_feb += el_niño # Add in a climate effect
&gt;&gt;&gt; temperature_feb
NormalDist(mu=10.0, sigma=6.830080526611674)

&gt;&gt;&gt; temperature_feb * (9/5) + 32 # Convert to Fahrenheit
NormalDist(mu=50.0, sigma=12.294144947901014)
&gt;&gt;&gt; temperature_feb.samples(3) # Generate random samples
[7.672102882379219, 12.000027119750287, 4.647488369766392]
</pre></div>
</div>
</section>
<section id="sys">
<h3>sys<a class="headerlink" href="#sys" title="Permalink to this headline">¶</a></h3>
Add new <a class="reference internal" href="../library/sys.html#sys.unraisablehook" title="sys.unraisablehook"><code class="xref py py-func docutils literal notranslate">sys.unraisablehook()</code></a> function which can be overridden to control
how “unraisable exceptions” are handled. It is called when an exception has
occurred but there is no way for Python to handle it. For example, when a
destructor raises an exception or during garbage collection
(<a class="reference internal" href="../library/gc.html#gc.collect" title="gc.collect"><code class="xref py py-func docutils literal notranslate">gc.collect()</code></a>).
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36829">bpo-36829</a>.)
</section>
<section id="tarfile">
<h3>tarfile<a class="headerlink" href="#tarfile" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/tarfile.html#module-tarfile" title="tarfile: Read and write tar-format archive files."><code class="xref py py-mod docutils literal notranslate">tarfile</code></a> module now defaults to the modern pax (POSIX.1-2001)
format for new archives, instead of the previous GNU-specific one.
This improves cross-platform portability with a consistent encoding (UTF-8)
in a standardized and extensible format, and offers several other benefits.
(Contributed by C.A.M. Gerlach in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36268">bpo-36268</a>.)
</section>
<section id="threading">
<h3>threading<a class="headerlink" href="#threading" title="Permalink to this headline">¶</a></h3>
Add a new <a class="reference internal" href="../library/threading.html#threading.excepthook" title="threading.excepthook"><code class="xref py py-func docutils literal notranslate">threading.excepthook()</code></a> function which handles uncaught
<a class="reference internal" href="../library/threading.html#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate">threading.Thread.run()</code></a> exception. It can be overridden to control how
uncaught <a class="reference internal" href="../library/threading.html#threading.Thread.run" title="threading.Thread.run"><code class="xref py py-meth docutils literal notranslate">threading.Thread.run()</code></a> exceptions are handled.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=1230540">bpo-1230540</a>.)
Add a new <a class="reference internal" href="../library/threading.html#threading.get_native_id" title="threading.get_native_id"><code class="xref py py-func docutils literal notranslate">threading.get_native_id()</code></a> function and
a <a class="reference internal" href="../library/threading.html#threading.Thread.native_id" title="threading.Thread.native_id"><code class="xref py py-data docutils literal notranslate">native_id</code></a>
attribute to the <a class="reference internal" href="../library/threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate">threading.Thread</code></a> class. These return the native
integral Thread ID of the current thread assigned by the kernel.
This feature is only available on certain platforms, see
<a class="reference internal" href="../library/threading.html#threading.get_native_id" title="threading.get_native_id"><code class="xref py py-func docutils literal notranslate">get_native_id</code></a> for more information.
(Contributed by Jake Tesler in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36084">bpo-36084</a>.)
</section>
<section id="tokenize">
<h3>tokenize<a class="headerlink" href="#tokenize" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/tokenize.html#module-tokenize" title="tokenize: Lexical scanner for Python source code."><code class="xref py py-mod docutils literal notranslate">tokenize</code></a> module now implicitly emits a <code class="docutils literal notranslate">NEWLINE</code> token when
provided with input that does not have a trailing new line. This behavior
now matches what the C tokenizer does internally.
(Contributed by Ammar Askar in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33899">bpo-33899</a>.)
</section>
<section id="tkinter">
<h3>tkinter<a class="headerlink" href="#tkinter" title="Permalink to this headline">¶</a></h3>
Added methods <code class="xref py py-meth docutils literal notranslate">selection_from()</code>,
<code class="xref py py-meth docutils literal notranslate">selection_present()</code>,
<code class="xref py py-meth docutils literal notranslate">selection_range()</code> and
<code class="xref py py-meth docutils literal notranslate">selection_to()</code>
in the <code class="xref py py-class docutils literal notranslate">tkinter.Spinbox</code> class.
(Contributed by Juliette Monsel in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34829">bpo-34829</a>.)
Added method <code class="xref py py-meth docutils literal notranslate">moveto()</code>
in the <code class="xref py py-class docutils literal notranslate">tkinter.Canvas</code> class.
(Contributed by Juliette Monsel in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=23831">bpo-23831</a>.)
The <code class="xref py py-class docutils literal notranslate">tkinter.PhotoImage</code> class now has
<code class="xref py py-meth docutils literal notranslate">transparency_get()</code> and
<code class="xref py py-meth docutils literal notranslate">transparency_set()</code> methods. (Contributed by
Zackery Spytz in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=25451">bpo-25451</a>.)
</section>
<section id="time">
<h3>time<a class="headerlink" href="#time" title="Permalink to this headline">¶</a></h3>
Added new clock <a class="reference internal" href="../library/time.html#time.CLOCK_UPTIME_RAW" title="time.CLOCK_UPTIME_RAW"><code class="xref py py-data docutils literal notranslate">CLOCK_UPTIME_RAW</code></a> for macOS 10.12.
(Contributed by Joannah Nanjekye in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35702">bpo-35702</a>.)
</section>
<section id="typing">
<h3>typing<a class="headerlink" href="#typing" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/typing.html#module-typing" title="typing: Support for type hints (see :pep:`484`)."><code class="xref py py-mod docutils literal notranslate">typing</code></a> module incorporates several new features:
<ul>
<li>A dictionary type with per-key types. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0589">PEP 589</a> and
<a class="reference internal" href="../library/typing.html#typing.TypedDict" title="typing.TypedDict"><code class="xref py py-class docutils literal notranslate">typing.TypedDict</code></a>.
TypedDict uses only string keys. By default, every key is required
to be present. Specify “total=False” to allow keys to be optional:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>class Location(TypedDict, total=False):
 lat_long: tuple
 grid_square: str
 xy_coordinate: tuple
</pre></div>
</div>
</li>
<li>Literal types. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0586">PEP 586</a> and <a class="reference internal" href="../library/typing.html#typing.Literal" title="typing.Literal"><code class="xref py py-class docutils literal notranslate">typing.Literal</code></a>.
Literal types indicate that a parameter or return value
is constrained to one or more specific literal values:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>def get_status(port: int) -&gt; Literal[&#39;connected&#39;, &#39;disconnected&#39;]:
 ...
</pre></div>
</div>
</li>
<li>“Final” variables, functions, methods and classes. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0591">PEP 591</a>,
<a class="reference internal" href="../library/typing.html#typing.Final" title="typing.Final"><code class="xref py py-class docutils literal notranslate">typing.Final</code></a> and <a class="reference internal" href="../library/typing.html#typing.final" title="typing.final"><code class="xref py py-func docutils literal notranslate">typing.final()</code></a>.
The final qualifier instructs a static type checker to restrict
subclassing, overriding, or reassignment:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>pi: Final[float] = 3.1415926536
</pre></div>
</div>
</li>
<li>Protocol definitions. See <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0544">PEP 544</a>, <a class="reference internal" href="../library/typing.html#typing.Protocol" title="typing.Protocol"><code class="xref py py-class docutils literal notranslate">typing.Protocol</code></a> and
<a class="reference internal" href="../library/typing.html#typing.runtime_checkable" title="typing.runtime_checkable"><code class="xref py py-func docutils literal notranslate">typing.runtime_checkable()</code></a>. Simple ABCs like
<a class="reference internal" href="../library/typing.html#typing.SupportsInt" title="typing.SupportsInt"><code class="xref py py-class docutils literal notranslate">typing.SupportsInt</code></a> are now <code class="docutils literal notranslate">Protocol</code> subclasses.</li>
<li>New protocol class <a class="reference internal" href="../library/typing.html#typing.SupportsIndex" title="typing.SupportsIndex"><code class="xref py py-class docutils literal notranslate">typing.SupportsIndex</code></a>.</li>
<li>New functions <a class="reference internal" href="../library/typing.html#typing.get_origin" title="typing.get_origin"><code class="xref py py-func docutils literal notranslate">typing.get_origin()</code></a> and <a class="reference internal" href="../library/typing.html#typing.get_args" title="typing.get_args"><code class="xref py py-func docutils literal notranslate">typing.get_args()</code></a>.</li>
</ul>
</section>
<section id="unicodedata">
<h3>unicodedata<a class="headerlink" href="#unicodedata" title="Permalink to this headline">¶</a></h3>
The <a class="reference internal" href="../library/unicodedata.html#module-unicodedata" title="unicodedata: Access the Unicode Database."><code class="xref py py-mod docutils literal notranslate">unicodedata</code></a> module has been upgraded to use the <a class="reference external" href="https://blog.unicode.org/2019/05/unicode-12-1-en.html">Unicode 12.1.0</a> release.
New function <a class="reference internal" href="../library/unicodedata.html#unicodedata.is_normalized" title="unicodedata.is_normalized"><code class="xref py py-func docutils literal notranslate">is_normalized()</code></a> can be used to verify a string
is in a specific normal form, often much faster than by actually normalizing
the string. (Contributed by Max Belanger, David Euresti, and Greg Price in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32285">bpo-32285</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37966">bpo-37966</a>).
</section>
<section id="unittest">
<h3>unittest<a class="headerlink" href="#unittest" title="Permalink to this headline">¶</a></h3>
Added <a class="reference internal" href="../library/unittest.mock.html#unittest.mock.AsyncMock" title="unittest.mock.AsyncMock"><code class="xref py py-class docutils literal notranslate">AsyncMock</code></a> to support an asynchronous version of
<a class="reference internal" href="../library/unittest.mock.html#unittest.mock.Mock" title="unittest.mock.Mock"><code class="xref py py-class docutils literal notranslate">Mock</code></a>. Appropriate new assert functions for testing
have been added as well.
(Contributed by Lisa Roach in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=26467">bpo-26467</a>).
Added <a class="reference internal" href="../library/unittest.html#unittest.addModuleCleanup" title="unittest.addModuleCleanup"><code class="xref py py-func docutils literal notranslate">addModuleCleanup()</code></a> and
<a class="reference internal" href="../library/unittest.html#unittest.TestCase.addClassCleanup" title="unittest.TestCase.addClassCleanup"><code class="xref py py-meth docutils literal notranslate">addClassCleanup()</code></a> to unittest to support
cleanups for <code class="xref py py-func docutils literal notranslate">setUpModule()</code> and
<a class="reference internal" href="../library/unittest.html#unittest.TestCase.setUpClass" title="unittest.TestCase.setUpClass"><code class="xref py py-meth docutils literal notranslate">setUpClass()</code></a>.
(Contributed by Lisa Roach in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=24412">bpo-24412</a>.)
Several mock assert functions now also print a list of actual calls upon
failure. (Contributed by Petter Strandmark in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35047">bpo-35047</a>.)
<a class="reference internal" href="../library/unittest.html#module-unittest" title="unittest: Unit testing framework for Python."><code class="xref py py-mod docutils literal notranslate">unittest</code></a> module gained support for coroutines to be used as test cases
with <a class="reference internal" href="../library/unittest.html#unittest.IsolatedAsyncioTestCase" title="unittest.IsolatedAsyncioTestCase"><code class="xref py py-class docutils literal notranslate">unittest.IsolatedAsyncioTestCase</code></a>.
(Contributed by Andrew Svetlov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32972">bpo-32972</a>.)
Example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import unittest

class TestRequest(unittest.IsolatedAsyncioTestCase):

async def asyncSetUp(self):
 self.connection = await AsyncConnection()

async def test_get(self):
 response = await self.connection.get(&quot;https://example.com&quot;)
 self.assertEqual(response.status_code, 200)

async def asyncTearDown(self):
 await self.connection.close()

if __name__ == &quot;__main__&quot;:
 unittest.main()
</pre></div>
</div>
</section>
<section id="venv">
<h3>venv<a class="headerlink" href="#venv" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/venv.html#module-venv" title="venv: Creation of virtual environments."><code class="xref py py-mod docutils literal notranslate">venv</code></a> now includes an <code class="docutils literal notranslate">Activate.ps1</code> script on all platforms for
activating virtual environments under PowerShell Core 6.1.
(Contributed by Brett Cannon in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32718">bpo-32718</a>.)
</section>
<section id="weakref">
<h3>weakref<a class="headerlink" href="#weakref" title="Permalink to this headline">¶</a></h3>
The proxy objects returned by <a class="reference internal" href="../library/weakref.html#weakref.proxy" title="weakref.proxy"><code class="xref py py-func docutils literal notranslate">weakref.proxy()</code></a> now support the matrix
multiplication operators <code class="docutils literal notranslate">&#64;</code> and <code class="docutils literal notranslate">&#64;=</code> in addition to the other
numeric operators. (Contributed by Mark Dickinson in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36669">bpo-36669</a>.)
</section>
<section id="xml">
<h3>xml<a class="headerlink" href="#xml" title="Permalink to this headline">¶</a></h3>
As mitigation against DTD and external entity retrieval, the
<a class="reference internal" href="../library/xml.dom.minidom.html#module-xml.dom.minidom" title="xml.dom.minidom: Minimal Document Object Model (DOM) implementation."><code class="xref py py-mod docutils literal notranslate">xml.dom.minidom</code></a> and <a class="reference internal" href="../library/xml.sax.html#module-xml.sax" title="xml.sax: Package containing SAX2 base classes and convenience functions."><code class="xref py py-mod docutils literal notranslate">xml.sax</code></a> modules no longer process
external entities by default.
(Contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=17239">bpo-17239</a>.)
The <code class="docutils literal notranslate">.find*()</code> methods in the <a class="reference internal" href="../library/xml.etree.elementtree.html#module-xml.etree.ElementTree" title="xml.etree.ElementTree: Implementation of the ElementTree API."><code class="xref py py-mod docutils literal notranslate">xml.etree.ElementTree</code></a> module
support wildcard searches like <code class="docutils literal notranslate">{*}tag</code> which ignores the namespace
and <code class="docutils literal notranslate">{namespace}*</code> which returns all tags in the given namespace.
(Contributed by Stefan Behnel in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=28238">bpo-28238</a>.)
The <a class="reference internal" href="../library/xml.etree.elementtree.html#module-xml.etree.ElementTree" title="xml.etree.ElementTree: Implementation of the ElementTree API."><code class="xref py py-mod docutils literal notranslate">xml.etree.ElementTree</code></a> module provides a new function
<code class="xref py py-func docutils literal notranslate">–xml.etree.ElementTree.canonicalize()</code> that implements C14N 2.0.
(Contributed by Stefan Behnel in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=13611">bpo-13611</a>.)
The target object of <a class="reference internal" href="../library/xml.etree.elementtree.html#xml.etree.ElementTree.XMLParser" title="xml.etree.ElementTree.XMLParser"><code class="xref py py-class docutils literal notranslate">xml.etree.ElementTree.XMLParser</code></a> can
receive namespace declaration events through the new callback methods
<code class="docutils literal notranslate">start_ns()</code> and <code class="docutils literal notranslate">end_ns()</code>. Additionally, the
<a class="reference internal" href="../library/xml.etree.elementtree.html#xml.etree.ElementTree.TreeBuilder" title="xml.etree.ElementTree.TreeBuilder"><code class="xref py py-class docutils literal notranslate">xml.etree.ElementTree.TreeBuilder</code></a> target can be configured
to process events about comments and processing instructions to include
them in the generated tree.
(Contributed by Stefan Behnel in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36676">bpo-36676</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36673">bpo-36673</a>.)
</section>
<section id="xmlrpc">
<h3>xmlrpc<a class="headerlink" href="#xmlrpc" title="Permalink to this headline">¶</a></h3>
<a class="reference internal" href="../library/xmlrpc.client.html#xmlrpc.client.ServerProxy" title="xmlrpc.client.ServerProxy"><code class="xref py py-class docutils literal notranslate">xmlrpc.client.ServerProxy</code></a> now supports an optional headers keyword
argument for a sequence of HTTP headers to be sent with each request. Among
other things, this makes it possible to upgrade from default basic
authentication to faster session authentication.
(Contributed by Cédric Krier in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35153">bpo-35153</a>.)
</section>
</section>
<section id="optimizations">
<h2>Optimizations<a class="headerlink" href="#optimizations" title="Permalink to this headline">¶</a></h2>
<ul>
<li>The <a class="reference internal" href="../library/subprocess.html#module-subprocess" title="subprocess: Subprocess management."><code class="xref py py-mod docutils literal notranslate">subprocess</code></a> module can now use the <a class="reference internal" href="../library/os.html#os.posix_spawn" title="os.posix_spawn"><code class="xref py py-func docutils literal notranslate">os.posix_spawn()</code></a> function
in some cases for better performance. Currently, it is only used on macOS
and Linux (using glibc 2.24 or newer) if all these conditions are met:
<ul class="simple">
<li>close_fds is false;</li>
<li>preexec_fn, pass_fds, cwd and start_new_session parameters
are not set;</li>
<li>the executable path contains a directory.</li>
</ul>
(Contributed by Joannah Nanjekye and Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35537">bpo-35537</a>.)
</li>
<li><a class="reference internal" href="../library/shutil.html#shutil.copyfile" title="shutil.copyfile"><code class="xref py py-func docutils literal notranslate">shutil.copyfile()</code></a>, <a class="reference internal" href="../library/shutil.html#shutil.copy" title="shutil.copy"><code class="xref py py-func docutils literal notranslate">shutil.copy()</code></a>, <a class="reference internal" href="../library/shutil.html#shutil.copy2" title="shutil.copy2"><code class="xref py py-func docutils literal notranslate">shutil.copy2()</code></a>,
<a class="reference internal" href="../library/shutil.html#shutil.copytree" title="shutil.copytree"><code class="xref py py-func docutils literal notranslate">shutil.copytree()</code></a> and <a class="reference internal" href="../library/shutil.html#shutil.move" title="shutil.move"><code class="xref py py-func docutils literal notranslate">shutil.move()</code></a> use platform-specific
“fast-copy” syscalls on Linux and macOS in order to copy the file
more efficiently.
“fast-copy” means that the copying operation occurs within the kernel,
avoiding the use of userspace buffers in Python as in
“<code class="docutils literal notranslate">outfd.write(infd.read())</code>”.
On Windows <a class="reference internal" href="../library/shutil.html#shutil.copyfile" title="shutil.copyfile"><code class="xref py py-func docutils literal notranslate">shutil.copyfile()</code></a> uses a bigger default buffer size (1 MiB
instead of 16 KiB) and a <a class="reference internal" href="../library/stdtypes.html#memoryview" title="memoryview"><code class="xref py py-func docutils literal notranslate">memoryview()</code></a>-based variant of
<a class="reference internal" href="../library/shutil.html#shutil.copyfileobj" title="shutil.copyfileobj"><code class="xref py py-func docutils literal notranslate">shutil.copyfileobj()</code></a> is used.
The speedup for copying a 512 MiB file within the same partition is about
+26% on Linux, +50% on macOS and +40% on Windows. Also, much less CPU cycles
are consumed.
See <a class="reference internal" href="../library/shutil.html#shutil-platform-dependent-efficient-copy-operations">Platform-dependent efficient copy operations</a> section.
(Contributed by Giampaolo Rodolà in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33671">bpo-33671</a>.)</li>
<li><a class="reference internal" href="../library/shutil.html#shutil.copytree" title="shutil.copytree"><code class="xref py py-func docutils literal notranslate">shutil.copytree()</code></a> uses <a class="reference internal" href="../library/os.html#os.scandir" title="os.scandir"><code class="xref py py-func docutils literal notranslate">os.scandir()</code></a> function and all copy
functions depending from it use cached <a class="reference internal" href="../library/os.html#os.stat" title="os.stat"><code class="xref py py-func docutils literal notranslate">os.stat()</code></a> values. The speedup
for copying a directory with 8000 files is around +9% on Linux, +20% on
Windows and +30% on a Windows SMB share. Also the number of <a class="reference internal" href="../library/os.html#os.stat" title="os.stat"><code class="xref py py-func docutils literal notranslate">os.stat()</code></a>
syscalls is reduced by 38% making <a class="reference internal" href="../library/shutil.html#shutil.copytree" title="shutil.copytree"><code class="xref py py-func docutils literal notranslate">shutil.copytree()</code></a> especially faster
on network filesystems. (Contributed by Giampaolo Rodolà in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33695">bpo-33695</a>.)</li>
<li>The default protocol in the <a class="reference internal" href="../library/pickle.html#module-pickle" title="pickle: Convert Python objects to streams of bytes and back."><code class="xref py py-mod docutils literal notranslate">pickle</code></a> module is now Protocol 4,
first introduced in Python 3.4. It offers better performance and smaller
size compared to Protocol 3 available since Python 3.0.</li>
<li>Removed one <a class="reference internal" href="../c-api/intro.html#c.Py_ssize_t" title="Py_ssize_t"><code class="xref c c-type docutils literal notranslate">Py_ssize_t</code></a> member from <code class="docutils literal notranslate">PyGC_Head</code>. All GC tracked
objects (e.g. tuple, list, dict) size is reduced 4 or 8 bytes.
(Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33597">bpo-33597</a>.)</li>
<li><a class="reference internal" href="../library/uuid.html#uuid.UUID" title="uuid.UUID"><code class="xref py py-class docutils literal notranslate">uuid.UUID</code></a> now uses <code class="docutils literal notranslate">__slots__</code> to reduce its memory footprint.
(Contributed by Wouter Bolsterlee and Tal Einat in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=30977">bpo-30977</a>)</li>
<li>Improved performance of <a class="reference internal" href="../library/operator.html#operator.itemgetter" title="operator.itemgetter"><code class="xref py py-func docutils literal notranslate">operator.itemgetter()</code></a> by 33%. Optimized
argument handling and added a fast path for the common case of a single
non-negative integer index into a tuple (which is the typical use case in
the standard library). (Contributed by Raymond Hettinger in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35664">bpo-35664</a>.)</li>
<li>Sped-up field lookups in <a class="reference internal" href="../library/collections.html#collections.namedtuple" title="collections.namedtuple"><code class="xref py py-func docutils literal notranslate">collections.namedtuple()</code></a>. They are now more
than two times faster, making them the fastest form of instance variable
lookup in Python. (Contributed by Raymond Hettinger, Pablo Galindo, and
Joe Jevnik, Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32492">bpo-32492</a>.)</li>
<li>The <a class="reference internal" href="../library/stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate">list</code></a> constructor does not overallocate the internal item buffer
if the input iterable has a known length (the input implements <code class="docutils literal notranslate">__len__</code>).
This makes the created list 12% smaller on average. (Contributed by
Raymond Hettinger and Pablo Galindo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33234">bpo-33234</a>.)</li>
<li>Doubled the speed of class variable writes. When a non-dunder attribute
was updated, there was an unnecessary call to update slots.
(Contributed by Stefan Behnel, Pablo Galindo Salgado, Raymond Hettinger,
Neil Schemenauer, and Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36012">bpo-36012</a>.)</li>
<li>Reduced an overhead of converting arguments passed to many builtin functions
and methods. This sped up calling some simple builtin functions and
methods up to 20–50%. (Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=23867">bpo-23867</a>,
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35582">bpo-35582</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36127">bpo-36127</a>.)</li>
<li><code class="docutils literal notranslate">LOAD_GLOBAL</code> instruction now uses new “per opcode cache” mechanism.
It is about 40% faster now. (Contributed by Yury Selivanov and Inada Naoki in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=26219">bpo-26219</a>.)</li>
</ul>
</section>
<section id="build-and-c-api-changes">
<h2>Build and C API Changes<a class="headerlink" href="#build-and-c-api-changes" title="Permalink to this headline">¶</a></h2>
<ul>
<li>Default <a class="reference internal" href="../library/sys.html#sys.abiflags" title="sys.abiflags"><code class="xref py py-data docutils literal notranslate">sys.abiflags</code></a> became an empty string: the <code class="docutils literal notranslate">m</code> flag for
pymalloc became useless (builds with and without pymalloc are ABI compatible)
and so has been removed. (Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36707">bpo-36707</a>.)
Example of changes:
<ul class="simple">
<li>Only <code class="docutils literal notranslate">python3.8</code> program is installed, <code class="docutils literal notranslate">python3.8m</code> program is gone.</li>
<li>Only <code class="docutils literal notranslate">python3.8-config</code> script is installed, <code class="docutils literal notranslate">python3.8m-config</code> script
is gone.</li>
<li>The <code class="docutils literal notranslate">m</code> flag has been removed from the suffix of dynamic library
filenames: extension modules in the standard library as well as those
produced and installed by third-party packages, like those downloaded from
PyPI. On Linux, for example, the Python 3.7 suffix
<code class="docutils literal notranslate">.cpython-37m-x86_64-linux-gnu.so</code> became
<code class="docutils literal notranslate">.cpython-38-x86_64-linux-gnu.so</code> in Python 3.8.</li>
</ul>
</li>
<li>The header files have been reorganized to better separate the different kinds
of APIs:
<ul class="simple">
<li><code class="docutils literal notranslate">Include/*.h</code> should be the portable public stable C API.</li>
<li><code class="docutils literal notranslate">Include/cpython/*.h</code> should be the unstable C API specific to CPython;
public API, with some private API prefixed by <code class="docutils literal notranslate">_Py</code> or <code class="docutils literal notranslate">_PY</code>.</li>
<li><code class="docutils literal notranslate">Include/internal/*.h</code> is the private internal C API very specific to
CPython. This API comes with no backward compatibility warranty and should
not be used outside CPython. It is only exposed for very specific needs
like debuggers and profiles which has to access to CPython internals
without calling functions. This API is now installed by <code class="docutils literal notranslate">make install</code>.</li>
</ul>
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35134">bpo-35134</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35081">bpo-35081</a>,
work initiated by Eric Snow in Python 3.7.)
</li>
<li>Some macros have been converted to static inline functions: parameter types
and return type are well defined, they don’t have issues specific to macros,
variables have a local scopes. Examples:
<ul class="simple">
<li><a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate">Py_INCREF()</code></a>, <a class="reference internal" href="../c-api/refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate">Py_DECREF()</code></a></li>
<li><a class="reference internal" href="../c-api/refcounting.html#c.Py_XINCREF" title="Py_XINCREF"><code class="xref c c-func docutils literal notranslate">Py_XINCREF()</code></a>, <a class="reference internal" href="../c-api/refcounting.html#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate">Py_XDECREF()</code></a></li>
<li><code class="xref c c-func docutils literal notranslate">PyObject_INIT()</code>, <code class="xref c c-func docutils literal notranslate">PyObject_INIT_VAR()</code></li>
<li>Private functions: <code class="xref c c-func docutils literal notranslate">_PyObject_GC_TRACK()</code>,
<code class="xref c c-func docutils literal notranslate">_PyObject_GC_UNTRACK()</code>, <code class="xref c c-func docutils literal notranslate">_Py_Dealloc()</code></li>
</ul>
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35059">bpo-35059</a>.)
</li>
<li>The <code class="xref c c-func docutils literal notranslate">PyByteArray_Init()</code> and <code class="xref c c-func docutils literal notranslate">PyByteArray_Fini()</code> functions have
been removed. They did nothing since Python 2.7.4 and Python 3.2.0, were
excluded from the limited API (stable ABI), and were not documented.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35713">bpo-35713</a>.)</li>
<li>The result of <code class="xref c c-func docutils literal notranslate">PyExceptionClass_Name()</code> is now of type
<code class="docutils literal notranslate">const char *</code> rather of <code class="docutils literal notranslate">char *</code>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33818">bpo-33818</a>.)</li>
<li>The duality of <code class="docutils literal notranslate">Modules/Setup.dist</code> and <code class="docutils literal notranslate">Modules/Setup</code> has been
removed. Previously, when updating the CPython source tree, one had
to manually copy <code class="docutils literal notranslate">Modules/Setup.dist</code> (inside the source tree) to
<code class="docutils literal notranslate">Modules/Setup</code> (inside the build tree) in order to reflect any changes
upstream. This was of a small benefit to packagers at the expense of
a frequent annoyance to developers following CPython development, as
forgetting to copy the file could produce build failures.
Now the build system always reads from <code class="docutils literal notranslate">Modules/Setup</code> inside the source
tree. People who want to customize that file are encouraged to maintain
their changes in a git fork of CPython or as patch files, as they would do
for any other change to the source tree.
(Contributed by Antoine Pitrou in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32430">bpo-32430</a>.)
</li>
<li>Functions that convert Python number to C integer like
<a class="reference internal" href="../c-api/long.html#c.PyLong_AsLong" title="PyLong_AsLong"><code class="xref c c-func docutils literal notranslate">PyLong_AsLong()</code></a> and argument parsing functions like
<a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate">PyArg_ParseTuple()</code></a> with integer converting format units like <code class="docutils literal notranslate">'i'</code>
will now use the <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate">__index__()</code></a> special method instead of
<a class="reference internal" href="../reference/datamodel.html#object.__int__" title="object.__int__"><code class="xref py py-meth docutils literal notranslate">__int__()</code></a>, if available. The deprecation warning will be
emitted for objects with the <code class="docutils literal notranslate">__int__()</code> method but without the
<code class="docutils literal notranslate">__index__()</code> method (like <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate">Decimal</code></a> and
<a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate">Fraction</code></a>). <a class="reference internal" href="../c-api/number.html#c.PyNumber_Check" title="PyNumber_Check"><code class="xref c c-func docutils literal notranslate">PyNumber_Check()</code></a> will now return
<code class="docutils literal notranslate">1</code> for objects implementing <code class="docutils literal notranslate">__index__()</code>.
<a class="reference internal" href="../c-api/number.html#c.PyNumber_Long" title="PyNumber_Long"><code class="xref c c-func docutils literal notranslate">PyNumber_Long()</code></a>, <a class="reference internal" href="../c-api/number.html#c.PyNumber_Float" title="PyNumber_Float"><code class="xref c c-func docutils literal notranslate">PyNumber_Float()</code></a> and
<a class="reference internal" href="../c-api/float.html#c.PyFloat_AsDouble" title="PyFloat_AsDouble"><code class="xref c c-func docutils literal notranslate">PyFloat_AsDouble()</code></a> also now use the <code class="docutils literal notranslate">__index__()</code> method if
available.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36048">bpo-36048</a> and <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=20092">bpo-20092</a>.)</li>
<li>Heap-allocated type objects will now increase their reference count
in <a class="reference internal" href="../c-api/allocation.html#c.PyObject_Init" title="PyObject_Init"><code class="xref c c-func docutils literal notranslate">PyObject_Init()</code></a> (and its parallel macro <code class="docutils literal notranslate">PyObject_INIT</code>)
instead of in <a class="reference internal" href="../c-api/type.html#c.PyType_GenericAlloc" title="PyType_GenericAlloc"><code class="xref c c-func docutils literal notranslate">PyType_GenericAlloc()</code></a>. Types that modify instance
allocation or deallocation may need to be adjusted.
(Contributed by Eddie Elizondo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35810">bpo-35810</a>.)</li>
<li>The new function <a class="reference internal" href="../c-api/code.html#c.PyCode_NewWithPosOnlyArgs" title="PyCode_NewWithPosOnlyArgs"><code class="xref c c-func docutils literal notranslate">PyCode_NewWithPosOnlyArgs()</code></a> allows to create
code objects like <a class="reference internal" href="../c-api/code.html#c.PyCode_New" title="PyCode_New"><code class="xref c c-func docutils literal notranslate">PyCode_New()</code></a>, but with an extra posonlyargcount
parameter for indicating the number of positional-only arguments.
(Contributed by Pablo Galindo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37221">bpo-37221</a>.)</li>
<li><a class="reference internal" href="../c-api/init.html#c.Py_SetPath" title="Py_SetPath"><code class="xref c c-func docutils literal notranslate">Py_SetPath()</code></a> now sets <a class="reference internal" href="../library/sys.html#sys.executable" title="sys.executable"><code class="xref py py-data docutils literal notranslate">sys.executable</code></a> to the program full
path (<a class="reference internal" href="../c-api/init.html#c.Py_GetProgramFullPath" title="Py_GetProgramFullPath"><code class="xref c c-func docutils literal notranslate">Py_GetProgramFullPath()</code></a>) rather than to the program name
(<a class="reference internal" href="../c-api/init.html#c.Py_GetProgramName" title="Py_GetProgramName"><code class="xref c c-func docutils literal notranslate">Py_GetProgramName()</code></a>).
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=38234">bpo-38234</a>.)</li>
</ul>
</section>
<section id="deprecated">
<h2>Deprecated<a class="headerlink" href="#deprecated" title="Permalink to this headline">¶</a></h2>
<ul>
<li>The distutils <code class="docutils literal notranslate">bdist_wininst</code> command is now deprecated, use
<code class="docutils literal notranslate">bdist_wheel</code> (wheel packages) instead.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37481">bpo-37481</a>.)</li>
<li>Deprecated methods <code class="docutils literal notranslate">getchildren()</code> and <code class="docutils literal notranslate">getiterator()</code> in
the <a class="reference internal" href="../library/xml.etree.elementtree.html#module-xml.etree.ElementTree" title="xml.etree.ElementTree: Implementation of the ElementTree API."><code class="xref py py-mod docutils literal notranslate">ElementTree</code></a> module now emit a
<a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> instead of <a class="reference internal" href="../library/exceptions.html#PendingDeprecationWarning" title="PendingDeprecationWarning"><code class="xref py py-exc docutils literal notranslate">PendingDeprecationWarning</code></a>.
They will be removed in Python 3.9.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=29209">bpo-29209</a>.)</li>
<li>Passing an object that is not an instance of
<a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate">concurrent.futures.ThreadPoolExecutor</code></a> to
<a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.loop.set_default_executor" title="asyncio.loop.set_default_executor"><code class="xref py py-meth docutils literal notranslate">loop.set_default_executor()</code></a> is
deprecated and will be prohibited in Python 3.9.
(Contributed by Elvis Pranskevichus in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34075">bpo-34075</a>.)</li>
<li>The <code class="xref py py-meth docutils literal notranslate">__getitem__()</code> methods of <a class="reference internal" href="../library/xml.dom.pulldom.html#xml.dom.pulldom.DOMEventStream" title="xml.dom.pulldom.DOMEventStream"><code class="xref py py-class docutils literal notranslate">xml.dom.pulldom.DOMEventStream</code></a>,
<a class="reference internal" href="../library/wsgiref.html#wsgiref.util.FileWrapper" title="wsgiref.util.FileWrapper"><code class="xref py py-class docutils literal notranslate">wsgiref.util.FileWrapper</code></a> and <a class="reference internal" href="../library/fileinput.html#fileinput.FileInput" title="fileinput.FileInput"><code class="xref py py-class docutils literal notranslate">fileinput.FileInput</code></a> have been
deprecated.
Implementations of these methods have been ignoring their index parameter,
and returning the next item instead.
(Contributed by Berker Peksag in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=9372">bpo-9372</a>.)
</li>
<li>The <a class="reference internal" href="../library/typing.html#typing.NamedTuple" title="typing.NamedTuple"><code class="xref py py-class docutils literal notranslate">typing.NamedTuple</code></a> class has deprecated the <code class="docutils literal notranslate">_field_types</code>
attribute in favor of the <code class="docutils literal notranslate">__annotations__</code> attribute which has the same
information. (Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36320">bpo-36320</a>.)</li>
<li><a class="reference internal" href="../library/ast.html#module-ast" title="ast: Abstract Syntax Tree classes and manipulation."><code class="xref py py-mod docutils literal notranslate">ast</code></a> classes <code class="docutils literal notranslate">Num</code>, <code class="docutils literal notranslate">Str</code>, <code class="docutils literal notranslate">Bytes</code>, <code class="docutils literal notranslate">NameConstant</code> and
<code class="docutils literal notranslate">Ellipsis</code> are considered deprecated and will be removed in future Python
versions. <a class="reference internal" href="../library/ast.html#ast.Constant" title="ast.Constant"><code class="xref py py-class docutils literal notranslate">Constant</code></a> should be used instead.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32892">bpo-32892</a>.)</li>
<li><a class="reference internal" href="../library/ast.html#ast.NodeVisitor" title="ast.NodeVisitor"><code class="xref py py-class docutils literal notranslate">ast.NodeVisitor</code></a> methods <code class="docutils literal notranslate">visit_Num()</code>, <code class="docutils literal notranslate">visit_Str()</code>,
<code class="docutils literal notranslate">visit_Bytes()</code>, <code class="docutils literal notranslate">visit_NameConstant()</code> and <code class="docutils literal notranslate">visit_Ellipsis()</code> are
deprecated now and will not be called in future Python versions.
Add the <code class="xref py py-meth docutils literal notranslate">visit_Constant()</code> method to handle all
constant nodes.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36917">bpo-36917</a>.)</li>
<li>The <a class="reference internal" href="../library/asyncio-task.html#asyncio.coroutine" title="asyncio.coroutine"><code class="xref py py-func docutils literal notranslate">asyncio.coroutine()</code></a> <a class="reference internal" href="../glossary.html#term-decorator">decorator</a> is deprecated and will be
removed in version 3.10. Instead of <code class="docutils literal notranslate">&#64;asyncio.coroutine</code>, use
<a class="reference internal" href="../reference/compound_stmts.html#async-def"><code class="xref std std-keyword docutils literal notranslate">async def</code></a> instead.
(Contributed by Andrew Svetlov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36921">bpo-36921</a>.)</li>
<li>In <a class="reference internal" href="../library/asyncio.html#module-asyncio" title="asyncio: Asynchronous I/O."><code class="xref py py-mod docutils literal notranslate">asyncio</code></a>, the explicit passing of a loop argument has been
deprecated and will be removed in version 3.10 for the following:
<a class="reference internal" href="../library/asyncio-task.html#asyncio.sleep" title="asyncio.sleep"><code class="xref py py-func docutils literal notranslate">asyncio.sleep()</code></a>, <a class="reference internal" href="../library/asyncio-task.html#asyncio.gather" title="asyncio.gather"><code class="xref py py-func docutils literal notranslate">asyncio.gather()</code></a>, <a class="reference internal" href="../library/asyncio-task.html#asyncio.shield" title="asyncio.shield"><code class="xref py py-func docutils literal notranslate">asyncio.shield()</code></a>,
<a class="reference internal" href="../library/asyncio-task.html#asyncio.wait_for" title="asyncio.wait_for"><code class="xref py py-func docutils literal notranslate">asyncio.wait_for()</code></a>, <a class="reference internal" href="../library/asyncio-task.html#asyncio.wait" title="asyncio.wait"><code class="xref py py-func docutils literal notranslate">asyncio.wait()</code></a>, <a class="reference internal" href="../library/asyncio-task.html#asyncio.as_completed" title="asyncio.as_completed"><code class="xref py py-func docutils literal notranslate">asyncio.as_completed()</code></a>,
<a class="reference internal" href="../library/asyncio-task.html#asyncio.Task" title="asyncio.Task"><code class="xref py py-class docutils literal notranslate">asyncio.Task</code></a>, <a class="reference internal" href="../library/asyncio-sync.html#asyncio.Lock" title="asyncio.Lock"><code class="xref py py-class docutils literal notranslate">asyncio.Lock</code></a>, <a class="reference internal" href="../library/asyncio-sync.html#asyncio.Event" title="asyncio.Event"><code class="xref py py-class docutils literal notranslate">asyncio.Event</code></a>,
<a class="reference internal" href="../library/asyncio-sync.html#asyncio.Condition" title="asyncio.Condition"><code class="xref py py-class docutils literal notranslate">asyncio.Condition</code></a>, <a class="reference internal" href="../library/asyncio-sync.html#asyncio.Semaphore" title="asyncio.Semaphore"><code class="xref py py-class docutils literal notranslate">asyncio.Semaphore</code></a>,
<a class="reference internal" href="../library/asyncio-sync.html#asyncio.BoundedSemaphore" title="asyncio.BoundedSemaphore"><code class="xref py py-class docutils literal notranslate">asyncio.BoundedSemaphore</code></a>, <a class="reference internal" href="../library/asyncio-queue.html#asyncio.Queue" title="asyncio.Queue"><code class="xref py py-class docutils literal notranslate">asyncio.Queue</code></a>,
<a class="reference internal" href="../library/asyncio-subprocess.html#asyncio.create_subprocess_exec" title="asyncio.create_subprocess_exec"><code class="xref py py-func docutils literal notranslate">asyncio.create_subprocess_exec()</code></a>, and
<a class="reference internal" href="../library/asyncio-subprocess.html#asyncio.create_subprocess_shell" title="asyncio.create_subprocess_shell"><code class="xref py py-func docutils literal notranslate">asyncio.create_subprocess_shell()</code></a>.</li>
<li>The explicit passing of coroutine objects to <a class="reference internal" href="../library/asyncio-task.html#asyncio.wait" title="asyncio.wait"><code class="xref py py-func docutils literal notranslate">asyncio.wait()</code></a> has been
deprecated and will be removed in version 3.11.
(Contributed by Yury Selivanov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34790">bpo-34790</a>.)</li>
<li>The following functions and methods are deprecated in the <a class="reference internal" href="../library/gettext.html#module-gettext" title="gettext: Multilingual internationalization services."><code class="xref py py-mod docutils literal notranslate">gettext</code></a>
module: <a class="reference internal" href="../library/gettext.html#gettext.lgettext" title="gettext.lgettext"><code class="xref py py-func docutils literal notranslate">lgettext()</code></a>, <a class="reference internal" href="../library/gettext.html#gettext.ldgettext" title="gettext.ldgettext"><code class="xref py py-func docutils literal notranslate">ldgettext()</code></a>,
<a class="reference internal" href="../library/gettext.html#gettext.lngettext" title="gettext.lngettext"><code class="xref py py-func docutils literal notranslate">lngettext()</code></a> and <a class="reference internal" href="../library/gettext.html#gettext.ldngettext" title="gettext.ldngettext"><code class="xref py py-func docutils literal notranslate">ldngettext()</code></a>.
They return encoded bytes, and it’s possible that you will get unexpected
Unicode-related exceptions if there are encoding problems with the
translated strings. It’s much better to use alternatives which return
Unicode strings in Python 3. These functions have been broken for a long time.
Function <a class="reference internal" href="../library/gettext.html#gettext.bind_textdomain_codeset" title="gettext.bind_textdomain_codeset"><code class="xref py py-func docutils literal notranslate">bind_textdomain_codeset()</code></a>, methods
<a class="reference internal" href="../library/gettext.html#gettext.NullTranslations.output_charset" title="gettext.NullTranslations.output_charset"><code class="xref py py-meth docutils literal notranslate">output_charset()</code></a> and
<a class="reference internal" href="../library/gettext.html#gettext.NullTranslations.set_output_charset" title="gettext.NullTranslations.set_output_charset"><code class="xref py py-meth docutils literal notranslate">set_output_charset()</code></a>, and the codeset
parameter of functions <a class="reference internal" href="../library/gettext.html#gettext.translation" title="gettext.translation"><code class="xref py py-func docutils literal notranslate">translation()</code></a> and
<a class="reference internal" href="../library/gettext.html#gettext.install" title="gettext.install"><code class="xref py py-func docutils literal notranslate">install()</code></a> are also deprecated, since they are only used for
the <code class="docutils literal notranslate">l*gettext()</code> functions.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33710">bpo-33710</a>.)
</li>
<li>The <code class="xref py py-meth docutils literal notranslate">isAlive()</code> method of <a class="reference internal" href="../library/threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate">threading.Thread</code></a>
has been deprecated.
(Contributed by Dong-hee Na in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35283">bpo-35283</a>.)</li>
<li>Many builtin and extension functions that take integer arguments will
now emit a deprecation warning for <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate">Decimal</code></a>s,
<a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate">Fraction</code></a>s and any other objects that can be converted
to integers only with a loss (e.g. that have the <a class="reference internal" href="../reference/datamodel.html#object.__int__" title="object.__int__"><code class="xref py py-meth docutils literal notranslate">__int__()</code></a>
method but do not have the <a class="reference internal" href="../reference/datamodel.html#object.__index__" title="object.__index__"><code class="xref py py-meth docutils literal notranslate">__index__()</code></a> method). In future
version they will be errors.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36048">bpo-36048</a>.)</li>
<li>Deprecated passing the following arguments as keyword arguments:
<ul class="simple">
<li>func in <a class="reference internal" href="../library/functools.html#functools.partialmethod" title="functools.partialmethod"><code class="xref py py-func docutils literal notranslate">functools.partialmethod()</code></a>, <a class="reference internal" href="../library/weakref.html#weakref.finalize" title="weakref.finalize"><code class="xref py py-func docutils literal notranslate">weakref.finalize()</code></a>,
<a class="reference internal" href="../library/profile.html#profile.Profile.runcall" title="profile.Profile.runcall"><code class="xref py py-meth docutils literal notranslate">profile.Profile.runcall()</code></a>, <code class="xref py py-meth docutils literal notranslate">cProfile.Profile.runcall()</code>,
<a class="reference internal" href="../library/bdb.html#bdb.Bdb.runcall" title="bdb.Bdb.runcall"><code class="xref py py-meth docutils literal notranslate">bdb.Bdb.runcall()</code></a>, <a class="reference internal" href="../library/trace.html#trace.Trace.runfunc" title="trace.Trace.runfunc"><code class="xref py py-meth docutils literal notranslate">trace.Trace.runfunc()</code></a> and
<a class="reference internal" href="../library/curses.html#curses.wrapper" title="curses.wrapper"><code class="xref py py-func docutils literal notranslate">curses.wrapper()</code></a>.</li>
<li>function in <a class="reference internal" href="../library/unittest.html#unittest.TestCase.addCleanup" title="unittest.TestCase.addCleanup"><code class="xref py py-meth docutils literal notranslate">unittest.TestCase.addCleanup()</code></a>.</li>
<li>fn in the <a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.Executor.submit" title="concurrent.futures.Executor.submit"><code class="xref py py-meth docutils literal notranslate">submit()</code></a> method of
<a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate">concurrent.futures.ThreadPoolExecutor</code></a> and
<a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate">concurrent.futures.ProcessPoolExecutor</code></a>.</li>
<li>callback in <a class="reference internal" href="../library/contextlib.html#contextlib.ExitStack.callback" title="contextlib.ExitStack.callback"><code class="xref py py-meth docutils literal notranslate">contextlib.ExitStack.callback()</code></a>,
<code class="xref py py-meth docutils literal notranslate">contextlib.AsyncExitStack.callback()</code> and
<a class="reference internal" href="../library/contextlib.html#contextlib.AsyncExitStack.push_async_callback" title="contextlib.AsyncExitStack.push_async_callback"><code class="xref py py-meth docutils literal notranslate">contextlib.AsyncExitStack.push_async_callback()</code></a>.</li>
<li>c and typeid in the <code class="xref py py-meth docutils literal notranslate">create()</code>
method of <code class="xref py py-class docutils literal notranslate">multiprocessing.managers.Server</code> and
<code class="xref py py-class docutils literal notranslate">multiprocessing.managers.SharedMemoryServer</code>.</li>
<li>obj in <a class="reference internal" href="../library/weakref.html#weakref.finalize" title="weakref.finalize"><code class="xref py py-func docutils literal notranslate">weakref.finalize()</code></a>.</li>
</ul>
In future releases of Python, they will be <a class="reference internal" href="../glossary.html#positional-only-parameter">positional-only</a>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36492">bpo-36492</a>.)
</li>
</ul>
</section>
<section id="api-and-feature-removals">
<h2>API and Feature Removals<a class="headerlink" href="#api-and-feature-removals" title="Permalink to this headline">¶</a></h2>
The following features and APIs have been removed from Python 3.8:
<ul class="simple">
<li>Starting with Python 3.3, importing ABCs from <a class="reference internal" href="../library/collections.html#module-collections" title="collections: Container datatypes"><code class="xref py py-mod docutils literal notranslate">collections</code></a> was
deprecated, and importing should be done from <a class="reference internal" href="../library/collections.abc.html#module-collections.abc" title="collections.abc: Abstract base classes for containers"><code class="xref py py-mod docutils literal notranslate">collections.abc</code></a>. Being
able to import from collections was marked for removal in 3.8, but has been
delayed to 3.9. (See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36952">bpo-36952</a>.)</li>
<li>The <code class="xref py py-mod docutils literal notranslate">macpath</code> module, deprecated in Python 3.7, has been removed.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35471">bpo-35471</a>.)</li>
<li>The function <code class="xref py py-func docutils literal notranslate">platform.popen()</code> has been removed, after having been
deprecated since Python 3.3: use <a class="reference internal" href="../library/os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate">os.popen()</code></a> instead.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35345">bpo-35345</a>.)</li>
<li>The function <code class="xref py py-func docutils literal notranslate">time.clock()</code> has been removed, after having been
deprecated since Python 3.3: use <a class="reference internal" href="../library/time.html#time.perf_counter" title="time.perf_counter"><code class="xref py py-func docutils literal notranslate">time.perf_counter()</code></a> or
<a class="reference internal" href="../library/time.html#time.process_time" title="time.process_time"><code class="xref py py-func docutils literal notranslate">time.process_time()</code></a> instead, depending
on your requirements, to have well-defined behavior.
(Contributed by Matthias Bussonnier in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36895">bpo-36895</a>.)</li>
<li>The <code class="docutils literal notranslate">pyvenv</code> script has been removed in favor of <code class="docutils literal notranslate">python3.8 -m venv</code>
to help eliminate confusion as to what Python interpreter the <code class="docutils literal notranslate">pyvenv</code>
script is tied to. (Contributed by Brett Cannon in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=25427">bpo-25427</a>.)</li>
<li><code class="docutils literal notranslate">parse_qs</code>, <code class="docutils literal notranslate">parse_qsl</code>, and <code class="docutils literal notranslate">escape</code> are removed from the <a class="reference internal" href="../library/cgi.html#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface. (deprecated)"><code class="xref py py-mod docutils literal notranslate">cgi</code></a>
module. They are deprecated in Python 3.2 or older. They should be imported
from the <code class="docutils literal notranslate">urllib.parse</code> and <code class="docutils literal notranslate">html</code> modules instead.</li>
<li><code class="docutils literal notranslate">filemode</code> function is removed from the <a class="reference internal" href="../library/tarfile.html#module-tarfile" title="tarfile: Read and write tar-format archive files."><code class="xref py py-mod docutils literal notranslate">tarfile</code></a> module.
It is not documented and deprecated since Python 3.3.</li>
<li>The <a class="reference internal" href="../library/xml.etree.elementtree.html#xml.etree.ElementTree.XMLParser" title="xml.etree.ElementTree.XMLParser"><code class="xref py py-class docutils literal notranslate">XMLParser</code></a> constructor no longer accepts
the html argument. It never had an effect and was deprecated in Python 3.4.
All other parameters are now <a class="reference internal" href="../glossary.html#keyword-only-parameter">keyword-only</a>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=29209">bpo-29209</a>.)</li>
<li>Removed the <code class="docutils literal notranslate">doctype()</code> method of <a class="reference internal" href="../library/xml.etree.elementtree.html#xml.etree.ElementTree.XMLParser" title="xml.etree.ElementTree.XMLParser"><code class="xref py py-class docutils literal notranslate">XMLParser</code></a>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=29209">bpo-29209</a>.)</li>
<li>“unicode_internal” codec is removed.
(Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36297">bpo-36297</a>.)</li>
<li>The <code class="docutils literal notranslate">Cache</code> and <code class="docutils literal notranslate">Statement</code> objects of the <a class="reference internal" href="../library/sqlite3.html#module-sqlite3" title="sqlite3: A DB-API 2.0 implementation using SQLite 3.x."><code class="xref py py-mod docutils literal notranslate">sqlite3</code></a> module are not
exposed to the user.
(Contributed by Aviv Palivoda in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=30262">bpo-30262</a>.)</li>
<li>The <code class="docutils literal notranslate">bufsize</code> keyword argument of <a class="reference internal" href="../library/fileinput.html#fileinput.input" title="fileinput.input"><code class="xref py py-func docutils literal notranslate">fileinput.input()</code></a> and
<a class="reference internal" href="../library/fileinput.html#fileinput.FileInput" title="fileinput.FileInput"><code class="xref py py-func docutils literal notranslate">fileinput.FileInput()</code></a> which was ignored and deprecated since Python 3.6
has been removed. <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36952">bpo-36952</a> (Contributed by Matthias Bussonnier.)</li>
<li>The functions <code class="xref py py-func docutils literal notranslate">sys.set_coroutine_wrapper()</code> and
<code class="xref py py-func docutils literal notranslate">sys.get_coroutine_wrapper()</code> deprecated in Python 3.7 have been removed;
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36933">bpo-36933</a> (Contributed by Matthias Bussonnier.)</li>
</ul>
</section>
<section id="porting-to-python-3-8">
<h2>Porting to Python 3.8<a class="headerlink" href="#porting-to-python-3-8" title="Permalink to this headline">¶</a></h2>
This section lists previously described changes and other bugfixes
that may require changes to your code.
<section id="changes-in-python-behavior">
<h3>Changes in Python behavior<a class="headerlink" href="#changes-in-python-behavior" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li>Yield expressions (both <code class="docutils literal notranslate">yield</code> and <code class="docutils literal notranslate">yield from</code> clauses) are now disallowed
in comprehensions and generator expressions (aside from the iterable expression
in the leftmost <code class="xref std std-keyword docutils literal notranslate">for</code> clause).
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=10544">bpo-10544</a>.)</li>
<li>The compiler now produces a <a class="reference internal" href="../library/exceptions.html#SyntaxWarning" title="SyntaxWarning"><code class="xref py py-exc docutils literal notranslate">SyntaxWarning</code></a> when identity checks
(<code class="docutils literal notranslate">is</code> and <code class="docutils literal notranslate">is not</code>) are used with certain types of literals
(e.g. strings, numbers). These can often work by accident in CPython,
but are not guaranteed by the language spec. The warning advises users
to use equality tests (<code class="docutils literal notranslate">==</code> and <code class="docutils literal notranslate">!=</code>) instead.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34850">bpo-34850</a>.)</li>
<li>The CPython interpreter can swallow exceptions in some circumstances.
In Python 3.8 this happens in fewer cases. In particular, exceptions
raised when getting the attribute from the type dictionary are no longer
ignored. (Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35459">bpo-35459</a>.)</li>
<li>Removed <code class="docutils literal notranslate">__str__</code> implementations from builtin types <a class="reference internal" href="../library/functions.html#bool" title="bool"><code class="xref py py-class docutils literal notranslate">bool</code></a>,
<a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate">int</code></a>, <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate">float</code></a>, <a class="reference internal" href="../library/functions.html#complex" title="complex"><code class="xref py py-class docutils literal notranslate">complex</code></a> and few classes from
the standard library. They now inherit <code class="docutils literal notranslate">__str__()</code> from <a class="reference internal" href="../library/functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate">object</code></a>.
As result, defining the <code class="docutils literal notranslate">__repr__()</code> method in the subclass of these
classes will affect their string representation.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36793">bpo-36793</a>.)</li>
<li>On AIX, <a class="reference internal" href="../library/sys.html#sys.platform" title="sys.platform"><code class="xref py py-attr docutils literal notranslate">sys.platform</code></a> doesn’t contain the major version anymore.
It is always <code class="docutils literal notranslate">'aix'</code>, instead of <code class="docutils literal notranslate">'aix3'</code> .. <code class="docutils literal notranslate">'aix7'</code>. Since
older Python versions include the version number, so it is recommended to
always use <code class="docutils literal notranslate">sys.platform.startswith('aix')</code>.
(Contributed by M. Felt in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36588">bpo-36588</a>.)</li>
<li><a class="reference internal" href="../c-api/init.html#c.PyEval_AcquireLock" title="PyEval_AcquireLock"><code class="xref c c-func docutils literal notranslate">PyEval_AcquireLock()</code></a> and <a class="reference internal" href="../c-api/init.html#c.PyEval_AcquireThread" title="PyEval_AcquireThread"><code class="xref c c-func docutils literal notranslate">PyEval_AcquireThread()</code></a> now
terminate the current thread if called while the interpreter is
finalizing, making them consistent with <a class="reference internal" href="../c-api/init.html#c.PyEval_RestoreThread" title="PyEval_RestoreThread"><code class="xref c c-func docutils literal notranslate">PyEval_RestoreThread()</code></a>,
<a class="reference internal" href="../c-api/init.html#c.Py_END_ALLOW_THREADS" title="Py_END_ALLOW_THREADS"><code class="xref c c-func docutils literal notranslate">Py_END_ALLOW_THREADS()</code></a>, and <a class="reference internal" href="../c-api/init.html#c.PyGILState_Ensure" title="PyGILState_Ensure"><code class="xref c c-func docutils literal notranslate">PyGILState_Ensure()</code></a>. If this
behavior is not desired, guard the call by checking <code class="xref c c-func docutils literal notranslate">_Py_IsFinalizing()</code>
or <code class="xref c c-func docutils literal notranslate">sys.is_finalizing()</code>.
(Contributed by Joannah Nanjekye in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36475">bpo-36475</a>.)</li>
</ul>
</section>
<section id="changes-in-the-python-api">
<h3>Changes in the Python API<a class="headerlink" href="#changes-in-the-python-api" title="Permalink to this headline">¶</a></h3>
<ul class="simple">
<li>The <a class="reference internal" href="../library/os.html#os.getcwdb" title="os.getcwdb"><code class="xref py py-func docutils literal notranslate">os.getcwdb()</code></a> function now uses the UTF-8 encoding on Windows,
rather than the ANSI code page: see <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0529">PEP 529</a> for the rationale. The
function is no longer deprecated on Windows.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37412">bpo-37412</a>.)</li>
<li><a class="reference internal" href="../library/subprocess.html#subprocess.Popen" title="subprocess.Popen"><code class="xref py py-class docutils literal notranslate">subprocess.Popen</code></a> can now use <a class="reference internal" href="../library/os.html#os.posix_spawn" title="os.posix_spawn"><code class="xref py py-func docutils literal notranslate">os.posix_spawn()</code></a> in some cases
for better performance. On Windows Subsystem for Linux and QEMU User
Emulation, the <code class="xref py py-class docutils literal notranslate">Popen</code> constructor using <a class="reference internal" href="../library/os.html#os.posix_spawn" title="os.posix_spawn"><code class="xref py py-func docutils literal notranslate">os.posix_spawn()</code></a> no longer raises an
exception on errors like “missing program”. Instead the child process fails with a
non-zero <code class="xref py py-attr docutils literal notranslate">returncode</code>.
(Contributed by Joannah Nanjekye and Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35537">bpo-35537</a>.)</li>
<li>The preexec_fn argument of * <a class="reference internal" href="../library/subprocess.html#subprocess.Popen" title="subprocess.Popen"><code class="xref py py-class docutils literal notranslate">subprocess.Popen</code></a> is no longer
compatible with subinterpreters. The use of the parameter in a
subinterpreter now raises <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate">RuntimeError</code></a>.
(Contributed by Eric Snow in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34651">bpo-34651</a>, modified by Christian Heimes
in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37951">bpo-37951</a>.)</li>
<li>The <code class="xref py py-meth docutils literal notranslate">imap.IMAP4.logout()</code> method no longer silently ignores arbitrary
exceptions.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36348">bpo-36348</a>.)</li>
<li>The function <code class="xref py py-func docutils literal notranslate">platform.popen()</code> has been removed, after having been deprecated since
Python 3.3: use <a class="reference internal" href="../library/os.html#os.popen" title="os.popen"><code class="xref py py-func docutils literal notranslate">os.popen()</code></a> instead.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35345">bpo-35345</a>.)</li>
<li>The <a class="reference internal" href="../library/statistics.html#statistics.mode" title="statistics.mode"><code class="xref py py-func docutils literal notranslate">statistics.mode()</code></a> function no longer raises an exception
when given multimodal data. Instead, it returns the first mode
encountered in the input data. (Contributed by Raymond Hettinger
in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35892">bpo-35892</a>.)</li>
<li>The <a class="reference internal" href="../library/tkinter.ttk.html#tkinter.ttk.Treeview.selection" title="tkinter.ttk.Treeview.selection"><code class="xref py py-meth docutils literal notranslate">selection()</code></a> method of the
<a class="reference internal" href="../library/tkinter.ttk.html#tkinter.ttk.Treeview" title="tkinter.ttk.Treeview"><code class="xref py py-class docutils literal notranslate">tkinter.ttk.Treeview</code></a> class no longer takes arguments. Using it with
arguments for changing the selection was deprecated in Python 3.6. Use
specialized methods like <a class="reference internal" href="../library/tkinter.ttk.html#tkinter.ttk.Treeview.selection_set" title="tkinter.ttk.Treeview.selection_set"><code class="xref py py-meth docutils literal notranslate">selection_set()</code></a> for
changing the selection. (Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=31508">bpo-31508</a>.)</li>
<li>The <code class="xref py py-meth docutils literal notranslate">writexml()</code>, <code class="xref py py-meth docutils literal notranslate">toxml()</code> and <code class="xref py py-meth docutils literal notranslate">toprettyxml()</code> methods of
<a class="reference internal" href="../library/xml.dom.minidom.html#module-xml.dom.minidom" title="xml.dom.minidom: Minimal Document Object Model (DOM) implementation."><code class="xref py py-mod docutils literal notranslate">xml.dom.minidom</code></a>, and the <code class="xref py py-meth docutils literal notranslate">write()</code> method of <code class="xref py py-mod docutils literal notranslate">xml.etree</code>,
now preserve the attribute order specified by the user.
(Contributed by Diego Rojas and Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=34160">bpo-34160</a>.)</li>
<li>A <a class="reference internal" href="../library/dbm.html#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate">dbm.dumb</code></a> database opened with flags <code class="docutils literal notranslate">'r'</code> is now read-only.
<a class="reference internal" href="../library/dbm.html#dbm.dumb.open" title="dbm.dumb.open"><code class="xref py py-func docutils literal notranslate">dbm.dumb.open()</code></a> with flags <code class="docutils literal notranslate">'r'</code> and <code class="docutils literal notranslate">'w'</code> no longer creates
a database if it does not exist.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32749">bpo-32749</a>.)</li>
<li>The <code class="docutils literal notranslate">doctype()</code> method defined in a subclass of
<a class="reference internal" href="../library/xml.etree.elementtree.html#xml.etree.ElementTree.XMLParser" title="xml.etree.ElementTree.XMLParser"><code class="xref py py-class docutils literal notranslate">XMLParser</code></a> will no longer be called and will
emit a <a class="reference internal" href="../library/exceptions.html#RuntimeWarning" title="RuntimeWarning"><code class="xref py py-exc docutils literal notranslate">RuntimeWarning</code></a> instead of a <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a>.
Define the <a class="reference internal" href="../library/xml.etree.elementtree.html#xml.etree.ElementTree.TreeBuilder.doctype" title="xml.etree.ElementTree.TreeBuilder.doctype"><code class="xref py py-meth docutils literal notranslate">doctype()</code></a>
method on a target for handling an XML doctype declaration.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=29209">bpo-29209</a>.)</li>
<li>A <a class="reference internal" href="../library/exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate">RuntimeError</code></a> is now raised when the custom metaclass doesn’t
provide the <code class="docutils literal notranslate">__classcell__</code> entry in the namespace passed to
<code class="docutils literal notranslate">type.__new__</code>. A <a class="reference internal" href="../library/exceptions.html#DeprecationWarning" title="DeprecationWarning"><code class="xref py py-exc docutils literal notranslate">DeprecationWarning</code></a> was emitted in Python
3.6–3.7. (Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=23722">bpo-23722</a>.)</li>
<li>The <code class="xref py py-class docutils literal notranslate">cProfile.Profile</code> class can now be used as a context
manager. (Contributed by Scott Sanderson in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=29235">bpo-29235</a>.)</li>
<li><a class="reference internal" href="../library/shutil.html#shutil.copyfile" title="shutil.copyfile"><code class="xref py py-func docutils literal notranslate">shutil.copyfile()</code></a>, <a class="reference internal" href="../library/shutil.html#shutil.copy" title="shutil.copy"><code class="xref py py-func docutils literal notranslate">shutil.copy()</code></a>, <a class="reference internal" href="../library/shutil.html#shutil.copy2" title="shutil.copy2"><code class="xref py py-func docutils literal notranslate">shutil.copy2()</code></a>,
<a class="reference internal" href="../library/shutil.html#shutil.copytree" title="shutil.copytree"><code class="xref py py-func docutils literal notranslate">shutil.copytree()</code></a> and <a class="reference internal" href="../library/shutil.html#shutil.move" title="shutil.move"><code class="xref py py-func docutils literal notranslate">shutil.move()</code></a> use platform-specific
“fast-copy” syscalls (see
<a class="reference internal" href="../library/shutil.html#shutil-platform-dependent-efficient-copy-operations">Platform-dependent efficient copy operations</a> section).</li>
<li><a class="reference internal" href="../library/shutil.html#shutil.copyfile" title="shutil.copyfile"><code class="xref py py-func docutils literal notranslate">shutil.copyfile()</code></a> default buffer size on Windows was changed from
16 KiB to 1 MiB.</li>
<li>The <code class="docutils literal notranslate">PyGC_Head</code> struct has changed completely. All code that touched the
struct member should be rewritten. (See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33597">bpo-33597</a>.)</li>
<li>The <a class="reference internal" href="../c-api/init.html#c.PyInterpreterState" title="PyInterpreterState"><code class="xref c c-type docutils literal notranslate">PyInterpreterState</code></a> struct has been moved into the “internal”
header files (specifically Include/internal/pycore_pystate.h). An
opaque <code class="docutils literal notranslate">PyInterpreterState</code> is still available as part of the public
API (and stable ABI). The docs indicate that none of the struct’s
fields are public, so we hope no one has been using them. However,
if you do rely on one or more of those private fields and have no
alternative then please open a BPO issue. We’ll work on helping
you adjust (possibly including adding accessor functions to the
public API). (See <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35886">bpo-35886</a>.)</li>
<li>The <a class="reference internal" href="../library/mmap.html#mmap.mmap.flush" title="mmap.mmap.flush"><code class="xref py py-meth docutils literal notranslate">mmap.flush()</code></a> method now returns <code class="docutils literal notranslate">None</code> on
success and raises an exception on error under all platforms. Previously,
its behavior was platform-dependent: a nonzero value was returned on success;
zero was returned on error under Windows. A zero value was returned on
success; an exception was raised on error under Unix.
(Contributed by Berker Peksag in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=2122">bpo-2122</a>.)</li>
<li><a class="reference internal" href="../library/xml.dom.minidom.html#module-xml.dom.minidom" title="xml.dom.minidom: Minimal Document Object Model (DOM) implementation."><code class="xref py py-mod docutils literal notranslate">xml.dom.minidom</code></a> and <a class="reference internal" href="../library/xml.sax.html#module-xml.sax" title="xml.sax: Package containing SAX2 base classes and convenience functions."><code class="xref py py-mod docutils literal notranslate">xml.sax</code></a> modules no longer process
external entities by default.
(Contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=17239">bpo-17239</a>.)</li>
<li>Deleting a key from a read-only <a class="reference internal" href="../library/dbm.html#module-dbm" title="dbm: Interfaces to various Unix &quot;database&quot; formats."><code class="xref py py-mod docutils literal notranslate">dbm</code></a> database (<a class="reference internal" href="../library/dbm.html#module-dbm.dumb" title="dbm.dumb: Portable implementation of the simple DBM interface."><code class="xref py py-mod docutils literal notranslate">dbm.dumb</code></a>,
<a class="reference internal" href="../library/dbm.html#module-dbm.gnu" title="dbm.gnu: GNU's reinterpretation of dbm. (Unix)"><code class="xref py py-mod docutils literal notranslate">dbm.gnu</code></a> or <a class="reference internal" href="../library/dbm.html#module-dbm.ndbm" title="dbm.ndbm: The standard &quot;database&quot; interface, based on ndbm. (Unix)"><code class="xref py py-mod docutils literal notranslate">dbm.ndbm</code></a>) raises <code class="xref py py-attr docutils literal notranslate">error</code> (<a class="reference internal" href="../library/dbm.html#dbm.dumb.error" title="dbm.dumb.error"><code class="xref py py-exc docutils literal notranslate">dbm.dumb.error</code></a>,
<a class="reference internal" href="../library/dbm.html#dbm.gnu.error" title="dbm.gnu.error"><code class="xref py py-exc docutils literal notranslate">dbm.gnu.error</code></a> or <a class="reference internal" href="../library/dbm.html#dbm.ndbm.error" title="dbm.ndbm.error"><code class="xref py py-exc docutils literal notranslate">dbm.ndbm.error</code></a>) instead of <a class="reference internal" href="../library/exceptions.html#KeyError" title="KeyError"><code class="xref py py-exc docutils literal notranslate">KeyError</code></a>.
(Contributed by Xiang Zhang in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33106">bpo-33106</a>.)</li>
<li>Simplified AST for literals. All constants will be represented as
<a class="reference internal" href="../library/ast.html#ast.Constant" title="ast.Constant"><code class="xref py py-class docutils literal notranslate">ast.Constant</code></a> instances. Instantiating old classes <code class="docutils literal notranslate">Num</code>,
<code class="docutils literal notranslate">Str</code>, <code class="docutils literal notranslate">Bytes</code>, <code class="docutils literal notranslate">NameConstant</code> and <code class="docutils literal notranslate">Ellipsis</code> will return
an instance of <code class="docutils literal notranslate">Constant</code>.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32892">bpo-32892</a>.)</li>
<li><a class="reference internal" href="../library/os.path.html#os.path.expanduser" title="os.path.expanduser"><code class="xref py py-func docutils literal notranslate">expanduser()</code></a> on Windows now prefers the <code class="xref std std-envvar docutils literal notranslate">USERPROFILE</code>
environment variable and does not use <code class="xref std std-envvar docutils literal notranslate">HOME</code>, which is not normally
set for regular user accounts.
(Contributed by Anthony Sottile in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36264">bpo-36264</a>.)</li>
<li>The exception <a class="reference internal" href="../library/asyncio-exceptions.html#asyncio.CancelledError" title="asyncio.CancelledError"><code class="xref py py-class docutils literal notranslate">asyncio.CancelledError</code></a> now inherits from
<a class="reference internal" href="../library/exceptions.html#BaseException" title="BaseException"><code class="xref py py-class docutils literal notranslate">BaseException</code></a> rather than <a class="reference internal" href="../library/exceptions.html#Exception" title="Exception"><code class="xref py py-class docutils literal notranslate">Exception</code></a> and no longer inherits
from <a class="reference internal" href="../library/concurrent.futures.html#concurrent.futures.CancelledError" title="concurrent.futures.CancelledError"><code class="xref py py-class docutils literal notranslate">concurrent.futures.CancelledError</code></a>.
(Contributed by Yury Selivanov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32528">bpo-32528</a>.)</li>
<li>The function <a class="reference internal" href="../library/asyncio-task.html#asyncio.wait_for" title="asyncio.wait_for"><code class="xref py py-func docutils literal notranslate">asyncio.wait_for()</code></a> now correctly waits for cancellation
when using an instance of <a class="reference internal" href="../library/asyncio-task.html#asyncio.Task" title="asyncio.Task"><code class="xref py py-class docutils literal notranslate">asyncio.Task</code></a>. Previously, upon reaching
timeout, it was cancelled and immediately returned.
(Contributed by Elvis Pranskevichus in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32751">bpo-32751</a>.)</li>
<li>The function <a class="reference internal" href="../library/asyncio-protocol.html#asyncio.BaseTransport.get_extra_info" title="asyncio.BaseTransport.get_extra_info"><code class="xref py py-func docutils literal notranslate">asyncio.BaseTransport.get_extra_info()</code></a> now returns a safe
to use socket object when ‘socket’ is passed to the name parameter.
(Contributed by Yury Selivanov in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37027">bpo-37027</a>.)</li>
<li><a class="reference internal" href="../library/asyncio-protocol.html#asyncio.BufferedProtocol" title="asyncio.BufferedProtocol"><code class="xref py py-class docutils literal notranslate">asyncio.BufferedProtocol</code></a> has graduated to the stable API.</li>
</ul>
<ul class="simple" id="bpo-36085-whatsnew">
<li>DLL dependencies for extension modules and DLLs loaded with <a class="reference internal" href="../library/ctypes.html#module-ctypes" title="ctypes: A foreign function library for Python."><code class="xref py py-mod docutils literal notranslate">ctypes</code></a> on
Windows are now resolved more securely. Only the system paths, the directory
containing the DLL or PYD file, and directories added with
<a class="reference internal" href="../library/os.html#os.add_dll_directory" title="os.add_dll_directory"><code class="xref py py-func docutils literal notranslate">add_dll_directory()</code></a> are searched for load-time dependencies.
Specifically, <code class="xref std std-envvar docutils literal notranslate">PATH</code> and the current working directory are no longer
used, and modifications to these will no longer have any effect on normal DLL
resolution. If your application relies on these mechanisms, you should check
for <a class="reference internal" href="../library/os.html#os.add_dll_directory" title="os.add_dll_directory"><code class="xref py py-func docutils literal notranslate">add_dll_directory()</code></a> and if it exists, use it to add your DLLs
directory while loading your library. Note that Windows 7 users will need to
ensure that Windows Update KB2533623 has been installed (this is also verified
by the installer).
(Contributed by Steve Dower in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36085">bpo-36085</a>.)</li>
<li>The header files and functions related to pgen have been removed after its
replacement by a pure Python implementation. (Contributed by Pablo Galindo
in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36623">bpo-36623</a>.)</li>
<li><a class="reference internal" href="../library/types.html#types.CodeType" title="types.CodeType"><code class="xref py py-class docutils literal notranslate">types.CodeType</code></a> has a new parameter in the second position of the
constructor (posonlyargcount) to support positional-only arguments defined
in <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0570">PEP 570</a>. The first argument (argcount) now represents the total
number of positional arguments (including positional-only arguments). The new
<code class="docutils literal notranslate">replace()</code> method of <a class="reference internal" href="../library/types.html#types.CodeType" title="types.CodeType"><code class="xref py py-class docutils literal notranslate">types.CodeType</code></a> can be used to make the code
future-proof.</li>
<li>The parameter <code class="docutils literal notranslate">digestmod</code> for <a class="reference internal" href="../library/hmac.html#hmac.new" title="hmac.new"><code class="xref py py-func docutils literal notranslate">hmac.new()</code></a> no longer uses the MD5 digest
by default.</li>
</ul>
</section>
<section id="changes-in-the-c-api">
<h3>Changes in the C API<a class="headerlink" href="#changes-in-the-c-api" title="Permalink to this headline">¶</a></h3>
<ul>
<li>The <a class="reference internal" href="../c-api/veryhigh.html#c.PyCompilerFlags" title="PyCompilerFlags"><code class="xref c c-struct docutils literal notranslate">PyCompilerFlags</code></a> structure got a new cf_feature_version
field. It should be initialized to <code class="docutils literal notranslate">PY_MINOR_VERSION</code>. The field is ignored
by default, and is used if and only if <code class="docutils literal notranslate">PyCF_ONLY_AST</code> flag is set in
cf_flags.
(Contributed by Guido van Rossum in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35766">bpo-35766</a>.)</li>
<li>The <code class="xref c c-func docutils literal notranslate">PyEval_ReInitThreads()</code> function has been removed from the C API.
It should not be called explicitly: use <a class="reference internal" href="../c-api/sys.html#c.PyOS_AfterFork_Child" title="PyOS_AfterFork_Child"><code class="xref c c-func docutils literal notranslate">PyOS_AfterFork_Child()</code></a>
instead.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36728">bpo-36728</a>.)</li>
<li>On Unix, C extensions are no longer linked to libpython except on Android
and Cygwin. When Python is embedded, <code class="docutils literal notranslate">libpython</code> must not be loaded with
<code class="docutils literal notranslate">RTLD_LOCAL</code>, but <code class="docutils literal notranslate">RTLD_GLOBAL</code> instead. Previously, using
<code class="docutils literal notranslate">RTLD_LOCAL</code>, it was already not possible to load C extensions which
were not linked to <code class="docutils literal notranslate">libpython</code>, like C extensions of the standard
library built by the <code class="docutils literal notranslate">*shared*</code> section of <code class="docutils literal notranslate">Modules/Setup</code>.
(Contributed by Victor Stinner in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=21536">bpo-21536</a>.)</li>
<li>Use of <code class="docutils literal notranslate">#</code> variants of formats in parsing or building value (e.g.
<a class="reference internal" href="../c-api/arg.html#c.PyArg_ParseTuple" title="PyArg_ParseTuple"><code class="xref c c-func docutils literal notranslate">PyArg_ParseTuple()</code></a>, <a class="reference internal" href="../c-api/arg.html#c.Py_BuildValue" title="Py_BuildValue"><code class="xref c c-func docutils literal notranslate">Py_BuildValue()</code></a>, <a class="reference internal" href="../c-api/call.html#c.PyObject_CallFunction" title="PyObject_CallFunction"><code class="xref c c-func docutils literal notranslate">PyObject_CallFunction()</code></a>,
etc.) without <code class="docutils literal notranslate">PY_SSIZE_T_CLEAN</code> defined raises <code class="docutils literal notranslate">DeprecationWarning</code> now.
It will be removed in 3.10 or 4.0. Read <a class="reference internal" href="../c-api/arg.html#arg-parsing">Parsing arguments and building values</a> for detail.
(Contributed by Inada Naoki in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36381">bpo-36381</a>.)</li>
<li>Instances of heap-allocated types (such as those created with
<a class="reference internal" href="../c-api/type.html#c.PyType_FromSpec" title="PyType_FromSpec"><code class="xref c c-func docutils literal notranslate">PyType_FromSpec()</code></a>) hold a reference to their type object.
Increasing the reference count of these type objects has been moved from
<a class="reference internal" href="../c-api/type.html#c.PyType_GenericAlloc" title="PyType_GenericAlloc"><code class="xref c c-func docutils literal notranslate">PyType_GenericAlloc()</code></a> to the more low-level functions,
<a class="reference internal" href="../c-api/allocation.html#c.PyObject_Init" title="PyObject_Init"><code class="xref c c-func docutils literal notranslate">PyObject_Init()</code></a> and <code class="xref c c-func docutils literal notranslate">PyObject_INIT()</code>.
This makes types created through <a class="reference internal" href="../c-api/type.html#c.PyType_FromSpec" title="PyType_FromSpec"><code class="xref c c-func docutils literal notranslate">PyType_FromSpec()</code></a> behave like
other classes in managed code.
<a class="reference internal" href="../c-api/typeobj.html#static-types">Statically allocated types</a> are not affected.
For the vast majority of cases, there should be no side effect.
However, types that manually increase the reference count after allocating
an instance (perhaps to work around the bug) may now become immortal.
To avoid this, these classes need to call Py_DECREF on the type object
during instance deallocation.
To correctly port these types into 3.8, please apply the following
changes:
<ul>
<li>Remove <a class="reference internal" href="../c-api/refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-macro docutils literal notranslate">Py_INCREF</code></a> on the type object after allocating an
instance - if any.
This may happen after calling <a class="reference internal" href="../c-api/allocation.html#c.PyObject_New" title="PyObject_New"><code class="xref c c-func docutils literal notranslate">PyObject_New()</code></a>,
<a class="reference internal" href="../c-api/allocation.html#c.PyObject_NewVar" title="PyObject_NewVar"><code class="xref c c-func docutils literal notranslate">PyObject_NewVar()</code></a>, <a class="reference internal" href="../c-api/gcsupport.html#c.PyObject_GC_New" title="PyObject_GC_New"><code class="xref c c-func docutils literal notranslate">PyObject_GC_New()</code></a>,
<a class="reference internal" href="../c-api/gcsupport.html#c.PyObject_GC_NewVar" title="PyObject_GC_NewVar"><code class="xref c c-func docutils literal notranslate">PyObject_GC_NewVar()</code></a>, or any other custom allocator that uses
<a class="reference internal" href="../c-api/allocation.html#c.PyObject_Init" title="PyObject_Init"><code class="xref c c-func docutils literal notranslate">PyObject_Init()</code></a> or <code class="xref c c-func docutils literal notranslate">PyObject_INIT()</code>.
Example:
<div class="highlight-c notranslate"><div class="highlight"><pre>static foo_struct *
foo_new(PyObject *type) {
 foo_struct *foo = PyObject_GC_New(foo_struct, (PyTypeObject *) type);
 if (foo == NULL)
 return NULL;
#if PY_VERSION_HEX &lt; 0x03080000
 // Workaround for Python issue 35810; no longer necessary in Python 3.8
 PY_INCREF(type)
#endif
 return foo;
}
</pre></div>
</div>
</li>
<li>Ensure that all custom <code class="docutils literal notranslate">tp_dealloc</code> functions of heap-allocated types
decrease the type’s reference count.
Example:
<div class="highlight-c notranslate"><div class="highlight"><pre>static void
foo_dealloc(foo_struct *instance) {
 PyObject *type = Py_TYPE(instance);
 PyObject_GC_Del(instance);
#if PY_VERSION_HEX &gt;= 0x03080000
 // This was not needed before Python 3.8 (Python issue 35810)
 Py_DECREF(type);
#endif
}
</pre></div>
</div>
</li>
</ul>
(Contributed by Eddie Elizondo in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35810">bpo-35810</a>.)
</li>
<li>The <a class="reference internal" href="../c-api/intro.html#c.Py_DEPRECATED" title="Py_DEPRECATED"><code class="xref c c-macro docutils literal notranslate">Py_DEPRECATED()</code></a> macro has been implemented for MSVC.
The macro now must be placed before the symbol name.
Example:
<div class="highlight-c notranslate"><div class="highlight"><pre>Py_DEPRECATED(3.8) PyAPI_FUNC(int) Py_OldFunction(void);
</pre></div>
</div>
(Contributed by Zackery Spytz in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33407">bpo-33407</a>.)
</li>
<li>The interpreter does not pretend to support binary compatibility of
extension types across feature releases, anymore. A <a class="reference internal" href="../c-api/type.html#c.PyTypeObject" title="PyTypeObject"><code class="xref c c-type docutils literal notranslate">PyTypeObject</code></a>
exported by a third-party extension module is supposed to have all the
slots expected in the current Python version, including
<a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_finalize" title="PyTypeObject.tp_finalize"><code class="xref c c-member docutils literal notranslate">tp_finalize</code></a> (<a class="reference internal" href="../c-api/typeobj.html#Py_TPFLAGS_HAVE_FINALIZE" title="Py_TPFLAGS_HAVE_FINALIZE"><code class="xref py py-const docutils literal notranslate">Py_TPFLAGS_HAVE_FINALIZE</code></a>
is not checked anymore before reading <a class="reference internal" href="../c-api/typeobj.html#c.PyTypeObject.tp_finalize" title="PyTypeObject.tp_finalize"><code class="xref c c-member docutils literal notranslate">tp_finalize</code></a>).
(Contributed by Antoine Pitrou in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=32388">bpo-32388</a>.)
</li>
<li>The functions <code class="xref c c-func docutils literal notranslate">PyNode_AddChild()</code> and <code class="xref c c-func docutils literal notranslate">PyParser_AddToken()</code> now accept
two additional <code class="docutils literal notranslate">int</code> arguments end_lineno and end_col_offset.</li>
<li>The <code class="file docutils literal notranslate">libpython38.a</code> file to allow MinGW tools to link directly against
<code class="file docutils literal notranslate">python38.dll</code> is no longer included in the regular Windows distribution.
If you require this file, it may be generated with the <code class="docutils literal notranslate">gendef</code> and
<code class="docutils literal notranslate">dlltool</code> tools, which are part of the MinGW binutils package:
<div class="highlight-shell notranslate"><div class="highlight"><pre>gendef - python38.dll &gt; tmp.def
dlltool --dllname python38.dll --def tmp.def --output-lib libpython38.a
</pre></div>
</div>
The location of an installed <code class="file docutils literal notranslate">pythonXY.dll</code> will depend on the
installation options and the version and language of Windows. See
<a class="reference internal" href="../using/windows.html#using-on-windows">Using Python on Windows</a> for more information. The resulting library should be
placed in the same directory as <code class="file docutils literal notranslate">pythonXY.lib</code>, which is generally the
<code class="file docutils literal notranslate">libs</code> directory under your Python installation.
(Contributed by Steve Dower in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37351">bpo-37351</a>.)
</li>
</ul>
</section>
<section id="cpython-bytecode-changes">
<h3>CPython bytecode changes<a class="headerlink" href="#cpython-bytecode-changes" title="Permalink to this headline">¶</a></h3>
<ul>
<li>The interpreter loop has been simplified by moving the logic of unrolling
the stack of blocks into the compiler. The compiler emits now explicit
instructions for adjusting the stack of values and calling the
cleaning-up code for <a class="reference internal" href="../reference/simple_stmts.html#break"><code class="xref std std-keyword docutils literal notranslate">break</code></a>, <a class="reference internal" href="../reference/simple_stmts.html#continue"><code class="xref std std-keyword docutils literal notranslate">continue</code></a> and
<a class="reference internal" href="../reference/simple_stmts.html#return"><code class="xref std std-keyword docutils literal notranslate">return</code></a>.
Removed opcodes <code class="xref std std-opcode docutils literal notranslate">BREAK_LOOP</code>, <code class="xref std std-opcode docutils literal notranslate">CONTINUE_LOOP</code>,
<code class="xref std std-opcode docutils literal notranslate">SETUP_LOOP</code> and <code class="xref std std-opcode docutils literal notranslate">SETUP_EXCEPT</code>. Added new opcodes
<a class="reference internal" href="../library/dis.html#opcode-ROT_FOUR"><code class="xref std std-opcode docutils literal notranslate">ROT_FOUR</code></a>, <code class="xref std std-opcode docutils literal notranslate">BEGIN_FINALLY</code>, <code class="xref std std-opcode docutils literal notranslate">CALL_FINALLY</code> and
<code class="xref std std-opcode docutils literal notranslate">POP_FINALLY</code>. Changed the behavior of <code class="xref std std-opcode docutils literal notranslate">END_FINALLY</code>
and <code class="xref std std-opcode docutils literal notranslate">WITH_CLEANUP_START</code>.
(Contributed by Mark Shannon, Antoine Pitrou and Serhiy Storchaka in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=17611">bpo-17611</a>.)
</li>
<li>Added new opcode <a class="reference internal" href="../library/dis.html#opcode-END_ASYNC_FOR"><code class="xref std std-opcode docutils literal notranslate">END_ASYNC_FOR</code></a> for handling exceptions raised
when awaiting a next item in an <a class="reference internal" href="../reference/compound_stmts.html#async-for"><code class="xref std std-keyword docutils literal notranslate">async for</code></a> loop.
(Contributed by Serhiy Storchaka in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=33041">bpo-33041</a>.)</li>
<li>The <a class="reference internal" href="../library/dis.html#opcode-MAP_ADD"><code class="xref std std-opcode docutils literal notranslate">MAP_ADD</code></a> now expects the value as the first element in the
stack and the key as the second element. This change was made so the key
is always evaluated before the value in dictionary comprehensions, as
proposed by <a class="pep reference external" href="https://www.python.org/dev/peps/pep-0572">PEP 572</a>. (Contributed by Jörn Heissler in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35224">bpo-35224</a>.)</li>
</ul>
</section>
<section id="demos-and-tools">
<h3>Demos and Tools<a class="headerlink" href="#demos-and-tools" title="Permalink to this headline">¶</a></h3>
Added a benchmark script for timing various ways to access variables:
<code class="docutils literal notranslate">Tools/scripts/var_access_benchmark.py</code>.
(Contributed by Raymond Hettinger in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=35884">bpo-35884</a>.)
Here’s a summary of performance improvements since Python 3.3:
<div class="highlight-none notranslate"><div class="highlight"><pre>Python version 3.3 3.4 3.5 3.6 3.7 3.8
-------------- --- --- --- --- --- ---

Variable and attribute read access:
    read_local                       4.0     7.1     7.1     5.4     5.1     3.9
    read_nonlocal                    5.3     7.1     8.1     5.8     5.4     4.4
    read_global                     13.3    15.5    19.0    14.3    13.6     7.6
    read_builtin                    20.0    21.1    21.6    18.5    19.0     7.5
    read_classvar_from_class        20.5    25.6    26.5    20.7    19.5    18.4
    read_classvar_from_instance     18.5    22.8    23.5    18.8    17.1    16.4
    read_instancevar                26.8    32.4    33.1    28.0    26.3    25.4
    read_instancevar_slots          23.7    27.8    31.3    20.8    20.8    20.2
    read_namedtuple                 68.5    73.8    57.5    45.0    46.8    18.4
    read_boundmethod                29.8    37.6    37.9    29.6    26.9    27.7

Variable and attribute write access:
    write_local                      4.6     8.7     9.3     5.5     5.3     4.3
    write_nonlocal                   7.3    10.5    11.1     5.6     5.5     4.7
    write_global                    15.9    19.7    21.2    18.0    18.0    15.8
    write_classvar                  81.9    92.9    96.0   104.6   102.1    39.2
    write_instancevar               36.4    44.6    45.8    40.0    38.9    35.5
    write_instancevar_slots         28.7    35.6    36.1    27.3    26.6    25.7

Data structure read access:
    read_list                       19.2    24.2    24.5    20.8    20.8    19.0
    read_deque                      19.9    24.7    25.5    20.2    20.6    19.8
    read_dict                       19.7    24.3    25.7    22.3    23.0    21.0
    read_strdict                    17.9    22.6    24.3    19.5    21.2    18.9

Data structure write access:
    write_list                      21.2    27.1    28.5    22.5    21.6    20.0
    write_deque                     23.8    28.7    30.1    22.7    21.8    23.5
    write_dict                      25.9    31.4    33.3    29.3    29.2    24.7
    write_strdict                   22.9    28.4    29.9    27.5    25.2    23.1

Stack (or queue) operations:
    list_append_pop                144.2    93.4   112.7    75.4    74.2    50.8
    deque_append_pop                30.4    43.5    57.0    49.4    49.2    42.5
    deque_append_popleft            30.8    43.7    57.3    49.7    49.7    42.8

Timing loop:
 loop_overhead 0.3 0.5 0.6 0.4 0.3 0.3
</pre></div>
</div>
The benchmarks were measured on an
<a class="reference external" href="https://ark.intel.com/content/www/us/en/ark/products/76088/intel-core-i7-4960hq-processor-6m-cache-up-to-3-80-ghz.html">Intel® Core™ i7-4960HQ processor</a>
running the macOS 64-bit builds found at
<a class="reference external" href="https://www.python.org/downloads/mac-osx/">python.org</a>.
The benchmark script displays timings in nanoseconds.
</section>
</section>
<section id="notable-changes-in-python-3-8-1">
<h2>Notable changes in Python 3.8.1<a class="headerlink" href="#notable-changes-in-python-3-8-1" title="Permalink to this headline">¶</a></h2>
Due to significant security concerns, the reuse_address parameter of
<a class="reference internal" href="../library/asyncio-eventloop.html#asyncio.loop.create_datagram_endpoint" title="asyncio.loop.create_datagram_endpoint"><code class="xref py py-meth docutils literal notranslate">asyncio.loop.create_datagram_endpoint()</code></a> is no longer supported. This is
because of the behavior of the socket option <code class="docutils literal notranslate">SO_REUSEADDR</code> in UDP. For more
details, see the documentation for <code class="docutils literal notranslate">loop.create_datagram_endpoint()</code>.
(Contributed by Kyle Stanley, Antoine Pitrou, and Yury Selivanov in
<a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=37228">bpo-37228</a>.)
</section>
<section id="notable-changes-in-python-3-8-8">
<h2>Notable changes in Python 3.8.8<a class="headerlink" href="#notable-changes-in-python-3-8-8" title="Permalink to this headline">¶</a></h2>
Earlier Python versions allowed using both <code class="docutils literal notranslate">;</code> and <code class="docutils literal notranslate">&amp;</code> as
query parameter separators in <a class="reference internal" href="../library/urllib.parse.html#urllib.parse.parse_qs" title="urllib.parse.parse_qs"><code class="xref py py-func docutils literal notranslate">urllib.parse.parse_qs()</code></a> and
<a class="reference internal" href="../library/urllib.parse.html#urllib.parse.parse_qsl" title="urllib.parse.parse_qsl"><code class="xref py py-func docutils literal notranslate">urllib.parse.parse_qsl()</code></a>. Due to security concerns, and to conform with
newer W3C recommendations, this has been changed to allow only a single
separator key, with <code class="docutils literal notranslate">&amp;</code> as the default. This change also affects
<a class="reference internal" href="../library/cgi.html#cgi.parse" title="cgi.parse"><code class="xref py py-func docutils literal notranslate">cgi.parse()</code></a> and <a class="reference internal" href="../library/cgi.html#cgi.parse_multipart" title="cgi.parse_multipart"><code class="xref py py-func docutils literal notranslate">cgi.parse_multipart()</code></a> as they use the affected
functions internally. For more details, please see their respective
documentation.
(Contributed by Adam Goldschmidt, Senthil Kumaran and Ken Jin in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=42967">bpo-42967</a>.)
</section>
<section id="notable-changes-in-python-3-8-12">
<h2>Notable changes in Python 3.8.12<a class="headerlink" href="#notable-changes-in-python-3-8-12" title="Permalink to this headline">¶</a></h2>
Starting with Python 3.8.12 the <a class="reference internal" href="../library/ipaddress.html#module-ipaddress" title="ipaddress: IPv4/IPv6 manipulation library."><code class="xref py py-mod docutils literal notranslate">ipaddress</code></a> module no longer accepts
any leading zeros in IPv4 address strings. Leading zeros are ambiguous and
interpreted as octal notation by some libraries. For example the legacy
function <a class="reference internal" href="../library/socket.html#socket.inet_aton" title="socket.inet_aton"><code class="xref py py-func docutils literal notranslate">socket.inet_aton()</code></a> treats leading zeros as octal notation.
glibc implementation of modern <a class="reference internal" href="../library/socket.html#socket.inet_pton" title="socket.inet_pton"><code class="xref py py-func docutils literal notranslate">inet_pton()</code></a> does not accept
any leading zeros.
(Originally contributed by Christian Heimes in <a class="reference external" href="https://bugs.python.org/issue?&#64;action=redirect&amp;bpo=36384">bpo-36384</a>, and backported
to 3.8 by Achraf Merzouki.)
</section>
</section>

<div class="clearer"></div>
 </div>
 </div>
 </div>
 <div class="sphinxsidebar" role="navigation" aria-label="main navigation">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">What’s New In Python 3.8</a><ul>
<li><a class="reference internal" href="#summary-release-highlights">Summary – Release highlights</a></li>
<li><a class="reference internal" href="#new-features">New Features</a><ul>
<li><a class="reference internal" href="#assignment-expressions">Assignment expressions</a></li>
<li><a class="reference internal" href="#positional-only-parameters">Positional-only parameters</a></li>
<li><a class="reference internal" href="#parallel-filesystem-cache-for-compiled-bytecode-files">Parallel filesystem cache for compiled bytecode files</a></li>
<li><a class="reference internal" href="#debug-build-uses-the-same-abi-as-release-build">Debug build uses the same ABI as release build</a></li>
<li><a class="reference internal" href="#f-strings-support-for-self-documenting-expressions-and-debugging">f-strings support <code class="docutils literal notranslate">=</code> for self-documenting expressions and debugging</a></li>
<li><a class="reference internal" href="#pep-578-python-runtime-audit-hooks">PEP 578: Python Runtime Audit Hooks</a></li>
<li><a class="reference internal" href="#pep-587-python-initialization-configuration">PEP 587: Python Initialization Configuration</a></li>
<li><a class="reference internal" href="#pep-590-vectorcall-a-fast-calling-protocol-for-cpython">PEP 590: Vectorcall: a fast calling protocol for CPython</a></li>
<li><a class="reference internal" href="#pickle-protocol-5-with-out-of-band-data-buffers">Pickle protocol 5 with out-of-band data buffers</a></li>
</ul>
</li>
<li><a class="reference internal" href="#other-language-changes">Other Language Changes</a></li>
<li><a class="reference internal" href="#new-modules">New Modules</a></li>
<li><a class="reference internal" href="#improved-modules">Improved Modules</a><ul>
<li><a class="reference internal" href="#ast">ast</a></li>
<li><a class="reference internal" href="#asyncio">asyncio</a></li>
<li><a class="reference internal" href="#builtins">builtins</a></li>
<li><a class="reference internal" href="#collections">collections</a></li>
<li><a class="reference internal" href="#cprofile">cProfile</a></li>
<li><a class="reference internal" href="#csv">csv</a></li>
<li><a class="reference internal" href="#curses">curses</a></li>
<li><a class="reference internal" href="#ctypes">ctypes</a></li>
<li><a class="reference internal" href="#datetime">datetime</a></li>
<li><a class="reference internal" href="#functools">functools</a></li>
<li><a class="reference internal" href="#gc">gc</a></li>
<li><a class="reference internal" href="#gettext">gettext</a></li>
<li><a class="reference internal" href="#gzip">gzip</a></li>
<li><a class="reference internal" href="#idle-and-idlelib">IDLE and idlelib</a></li>
<li><a class="reference internal" href="#inspect">inspect</a></li>
<li><a class="reference internal" href="#io">io</a></li>
<li><a class="reference internal" href="#itertools">itertools</a></li>
<li><a class="reference internal" href="#json-tool">json.tool</a></li>
<li><a class="reference internal" href="#logging">logging</a></li>
<li><a class="reference internal" href="#math">math</a></li>
<li><a class="reference internal" href="#mmap">mmap</a></li>
<li><a class="reference internal" href="#multiprocessing">multiprocessing</a></li>
<li><a class="reference internal" href="#os">os</a></li>
<li><a class="reference internal" href="#os-path">os.path</a></li>
<li><a class="reference internal" href="#pathlib">pathlib</a></li>
<li><a class="reference internal" href="#pickle">pickle</a></li>
<li><a class="reference internal" href="#plistlib">plistlib</a></li>
<li><a class="reference internal" href="#pprint">pprint</a></li>
<li><a class="reference internal" href="#py-compile">py_compile</a></li>
<li><a class="reference internal" href="#shlex">shlex</a></li>
<li><a class="reference internal" href="#shutil">shutil</a></li>
<li><a class="reference internal" href="#socket">socket</a></li>
<li><a class="reference internal" href="#ssl">ssl</a></li>
<li><a class="reference internal" href="#statistics">statistics</a></li>
<li><a class="reference internal" href="#sys">sys</a></li>
<li><a class="reference internal" href="#tarfile">tarfile</a></li>
<li><a class="reference internal" href="#threading">threading</a></li>
<li><a class="reference internal" href="#tokenize">tokenize</a></li>
<li><a class="reference internal" href="#tkinter">tkinter</a></li>
<li><a class="reference internal" href="#time">time</a></li>
<li><a class="reference internal" href="#typing">typing</a></li>
<li><a class="reference internal" href="#unicodedata">unicodedata</a></li>
<li><a class="reference internal" href="#unittest">unittest</a></li>
<li><a class="reference internal" href="#venv">venv</a></li>
<li><a class="reference internal" href="#weakref">weakref</a></li>
<li><a class="reference internal" href="#xml">xml</a></li>
<li><a class="reference internal" href="#xmlrpc">xmlrpc</a></li>
</ul>
</li>
<li><a class="reference internal" href="#optimizations">Optimizations</a></li>
<li><a class="reference internal" href="#build-and-c-api-changes">Build and C API Changes</a></li>
<li><a class="reference internal" href="#deprecated">Deprecated</a></li>
<li><a class="reference internal" href="#api-and-feature-removals">API and Feature Removals</a></li>
<li><a class="reference internal" href="#porting-to-python-3-8">Porting to Python 3.8</a><ul>
<li><a class="reference internal" href="#changes-in-python-behavior">Changes in Python behavior</a></li>
<li><a class="reference internal" href="#changes-in-the-python-api">Changes in the Python API</a></li>
<li><a class="reference internal" href="#changes-in-the-c-api">Changes in the C API</a></li>
<li><a class="reference internal" href="#cpython-bytecode-changes">CPython bytecode changes</a></li>
<li><a class="reference internal" href="#demos-and-tools">Demos and Tools</a></li>
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<li><a class="reference internal" href="#notable-changes-in-python-3-8-1">Notable changes in Python 3.8.1</a></li>
<li><a class="reference internal" href="#notable-changes-in-python-3-8-8">Notable changes in Python 3.8.8</a></li>
<li><a class="reference internal" href="#notable-changes-in-python-3-8-12">Notable changes in Python 3.8.12</a></li>
</ul>
</li>
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