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  <h3><a href="../contents.html">Table of Contents</a></h3>
  <ul>
<li><a class="reference internal" href="#">Annotations Best Practices</a><ul>
<li><a class="reference internal" href="#accessing-the-annotations-dict-of-an-object-in-python-3-10-and-newer">Accessing The Annotations Dict Of An Object In Python 3.10 And Newer</a></li>
<li><a class="reference internal" href="#accessing-the-annotations-dict-of-an-object-in-python-3-9-and-older">Accessing The Annotations Dict Of An Object In Python 3.9 And Older</a></li>
<li><a class="reference internal" href="#manually-un-stringizing-stringized-annotations">Manually Un-Stringizing Stringized Annotations</a></li>
<li><a class="reference internal" href="#best-practices-for-annotations-in-any-python-version">Best Practices For <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> In Any Python Version</a></li>
<li><a class="reference internal" href="#annotations-quirks"><code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> Quirks</a></li>
</ul>
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  <section id="annotations-best-practices">
<span id="annotations-howto"></span><h1>Annotations Best Practices<a class="headerlink" href="#annotations-best-practices" title="Permalink to this headline">¶</a></h1>
<dl class="field-list simple">
<dt class="field-odd">author</dt>
<dd class="field-odd"><p>Larry Hastings</p>
</dd>
</dl>
<div class="topic">
<p class="topic-title">Abstract</p>
<p>This document is designed to encapsulate the best practices
for working with annotations dicts.  If you write Python code
that examines <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> on Python objects, we
encourage you to follow the guidelines described below.</p>
<p>The document is organized into four sections:
best practices for accessing the annotations of an object
in Python versions 3.10 and newer,
best practices for accessing the annotations of an object
in Python versions 3.9 and older,
other best practices
for <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> that apply to any Python version,
and
quirks of <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code>.</p>
<p>Note that this document is specifically about working with
<code class="docutils literal notranslate"><span class="pre">__annotations__</span></code>, not uses <em>for</em> annotations.
If you’re looking for information on how to use “type hints”
in your code, please see 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"><span class="pre">typing</span></code></a> module.</p>
</div>
<section id="accessing-the-annotations-dict-of-an-object-in-python-3-10-and-newer">
<h2>Accessing The Annotations Dict Of An Object In Python 3.10 And Newer<a class="headerlink" href="#accessing-the-annotations-dict-of-an-object-in-python-3-10-and-newer" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div><p>Python 3.10 adds a new function to the standard library:
<a class="reference internal" href="../library/inspect.html#inspect.get_annotations" title="inspect.get_annotations"><code class="xref py py-func docutils literal notranslate"><span class="pre">inspect.get_annotations()</span></code></a>.  In Python versions 3.10
and newer, calling this function is the best practice for
accessing the annotations dict of any object that supports
annotations.  This function can also “un-stringize”
stringized annotations for you.</p>
<p>If for some reason <a class="reference internal" href="../library/inspect.html#inspect.get_annotations" title="inspect.get_annotations"><code class="xref py py-func docutils literal notranslate"><span class="pre">inspect.get_annotations()</span></code></a> isn’t
viable for your use case, you may access the
<code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> data member manually.  Best practice
for this changed in Python 3.10 as well: as of Python 3.10,
<code class="docutils literal notranslate"><span class="pre">o.__annotations__</span></code> is guaranteed to <em>always</em> work
on Python functions, classes, and modules.  If you’re
certain the object you’re examining is one of these three
<em>specific</em> objects, you may simply use <code class="docutils literal notranslate"><span class="pre">o.__annotations__</span></code>
to get at the object’s annotations dict.</p>
<p>However, other types of callables–for example,
callables created by <a class="reference internal" href="../library/functools.html#functools.partial" title="functools.partial"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.partial()</span></code></a>–may
not have an <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> attribute defined.  When
accessing the <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> of a possibly unknown
object,  best practice in Python versions 3.10 and
newer is to call <a class="reference internal" href="../library/functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate"><span class="pre">getattr()</span></code></a> with three arguments,
for example <code class="docutils literal notranslate"><span class="pre">getattr(o,</span> <span class="pre">'__annotations__',</span> <span class="pre">None)</span></code>.</p>
<p>Before Python 3.10, accessing <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> on a class that
defines no annotations but that has a parent class with
annotations would return the parent’s <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code>.
In Python 3.10 and newer, the child class’s annotations
will be an empty dict instead.</p>
</div></blockquote>
</section>
<section id="accessing-the-annotations-dict-of-an-object-in-python-3-9-and-older">
<h2>Accessing The Annotations Dict Of An Object In Python 3.9 And Older<a class="headerlink" href="#accessing-the-annotations-dict-of-an-object-in-python-3-9-and-older" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div><p>In Python 3.9 and older, accessing the annotations dict
of an object is much more complicated than in newer versions.
The problem is a design flaw in these older versions of Python,
specifically to do with class annotations.</p>
<p>Best practice for accessing the annotations dict of other
objects–functions, other callables, and modules–is the same
as best practice for 3.10, assuming you aren’t calling
<a class="reference internal" href="../library/inspect.html#inspect.get_annotations" title="inspect.get_annotations"><code class="xref py py-func docutils literal notranslate"><span class="pre">inspect.get_annotations()</span></code></a>: you should use three-argument
<a class="reference internal" href="../library/functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate"><span class="pre">getattr()</span></code></a> to access the object’s <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code>
attribute.</p>
<p>Unfortunately, this isn’t best practice for classes.  The problem
is that, since <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> is optional on classes, and
because classes can inherit attributes from their base classes,
accessing the <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> attribute of a class may
inadvertently return the annotations dict of a <em>base class.</em>
As an example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Base</span><span class="p">:</span>
    <span class="n">a</span><span class="p">:</span> <span class="nb">int</span> <span class="o">=</span> <span class="mi">3</span>
    <span class="n">b</span><span class="p">:</span> <span class="nb">str</span> <span class="o">=</span> <span class="s1">&#39;abc&#39;</span>

<span class="k">class</span> <span class="nc">Derived</span><span class="p">(</span><span class="n">Base</span><span class="p">):</span>
    <span class="k">pass</span>

<span class="nb">print</span><span class="p">(</span><span class="n">Derived</span><span class="o">.</span><span class="vm">__annotations__</span><span class="p">)</span>
</pre></div>
</div>
<p>This will print the annotations dict from <code class="docutils literal notranslate"><span class="pre">Base</span></code>, not
<code class="docutils literal notranslate"><span class="pre">Derived</span></code>.</p>
<p>Your code will have to have a separate code path if the object
you’re examining is a class (<code class="docutils literal notranslate"><span class="pre">isinstance(o,</span> <span class="pre">type)</span></code>).
In that case, best practice relies on an implementation detail
of Python 3.9 and before: if a class has annotations defined,
they are stored in the class’s <code class="docutils literal notranslate"><span class="pre">__dict__</span></code> dictionary.  Since
the class may or may not have annotations defined, best practice
is to call the <code class="docutils literal notranslate"><span class="pre">get</span></code> method on the class dict.</p>
<p>To put it all together, here is some sample code that safely
accesses the <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> attribute on an arbitrary
object in Python 3.9 and before:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">if</span> <span class="nb">isinstance</span><span class="p">(</span><span class="n">o</span><span class="p">,</span> <span class="nb">type</span><span class="p">):</span>
    <span class="n">ann</span> <span class="o">=</span> <span class="n">o</span><span class="o">.</span><span class="vm">__dict__</span><span class="o">.</span><span class="n">get</span><span class="p">(</span><span class="s1">&#39;__annotations__&#39;</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
<span class="k">else</span><span class="p">:</span>
    <span class="n">ann</span> <span class="o">=</span> <span class="nb">getattr</span><span class="p">(</span><span class="n">o</span><span class="p">,</span> <span class="s1">&#39;__annotations__&#39;</span><span class="p">,</span> <span class="kc">None</span><span class="p">)</span>
</pre></div>
</div>
<p>After running this code, <code class="docutils literal notranslate"><span class="pre">ann</span></code> should be either a
dictionary or <code class="docutils literal notranslate"><span class="pre">None</span></code>.  You’re encouraged to double-check
the type of <code class="docutils literal notranslate"><span class="pre">ann</span></code> using <a class="reference internal" href="../library/functions.html#isinstance" title="isinstance"><code class="xref py py-func docutils literal notranslate"><span class="pre">isinstance()</span></code></a> before further
examination.</p>
<p>Note that some exotic or malformed type objects may not have
a <code class="docutils literal notranslate"><span class="pre">__dict__</span></code> attribute, so for extra safety you may also wish
to use <a class="reference internal" href="../library/functions.html#getattr" title="getattr"><code class="xref py py-func docutils literal notranslate"><span class="pre">getattr()</span></code></a> to access <code class="docutils literal notranslate"><span class="pre">__dict__</span></code>.</p>
</div></blockquote>
</section>
<section id="manually-un-stringizing-stringized-annotations">
<h2>Manually Un-Stringizing Stringized Annotations<a class="headerlink" href="#manually-un-stringizing-stringized-annotations" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div><p>In situations where some annotations may be “stringized”,
and you wish to evaluate those strings to produce the
Python values they represent, it really is best to
call <a class="reference internal" href="../library/inspect.html#inspect.get_annotations" title="inspect.get_annotations"><code class="xref py py-func docutils literal notranslate"><span class="pre">inspect.get_annotations()</span></code></a> to do this work
for you.</p>
<p>If you’re using Python 3.9 or older, or if for some reason
you can’t use <a class="reference internal" href="../library/inspect.html#inspect.get_annotations" title="inspect.get_annotations"><code class="xref py py-func docutils literal notranslate"><span class="pre">inspect.get_annotations()</span></code></a>, you’ll need
to duplicate its logic.  You’re encouraged to examine the
implementation of <a class="reference internal" href="../library/inspect.html#inspect.get_annotations" title="inspect.get_annotations"><code class="xref py py-func docutils literal notranslate"><span class="pre">inspect.get_annotations()</span></code></a> in the
current Python version and follow a similar approach.</p>
<p>In a nutshell, if you wish to evaluate a stringized annotation
on an arbitrary object <code class="docutils literal notranslate"><span class="pre">o</span></code>:</p>
<ul class="simple">
<li><p>If <code class="docutils literal notranslate"><span class="pre">o</span></code> is a module, use <code class="docutils literal notranslate"><span class="pre">o.__dict__</span></code> as the
<code class="docutils literal notranslate"><span class="pre">globals</span></code> when calling <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a>.</p></li>
<li><p>If <code class="docutils literal notranslate"><span class="pre">o</span></code> is a class, use <code class="docutils literal notranslate"><span class="pre">sys.modules[o.__module__].__dict__</span></code>
as the <code class="docutils literal notranslate"><span class="pre">globals</span></code>, and <code class="docutils literal notranslate"><span class="pre">dict(vars(o))</span></code> as the <code class="docutils literal notranslate"><span class="pre">locals</span></code>,
when calling <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a>.</p></li>
<li><p>If <code class="docutils literal notranslate"><span class="pre">o</span></code> is a wrapped callable using <a class="reference internal" href="../library/functools.html#functools.update_wrapper" title="functools.update_wrapper"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.update_wrapper()</span></code></a>,
<a class="reference internal" href="../library/functools.html#functools.wraps" title="functools.wraps"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.wraps()</span></code></a>, or <a class="reference internal" href="../library/functools.html#functools.partial" title="functools.partial"><code class="xref py py-func docutils literal notranslate"><span class="pre">functools.partial()</span></code></a>, iteratively
unwrap it by accessing either <code class="docutils literal notranslate"><span class="pre">o.__wrapped__</span></code> or <code class="docutils literal notranslate"><span class="pre">o.func</span></code> as
appropriate, until you have found the root unwrapped function.</p></li>
<li><p>If <code class="docutils literal notranslate"><span class="pre">o</span></code> is a callable (but not a class), use
<code class="docutils literal notranslate"><span class="pre">o.__globals__</span></code> as the globals when calling <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a>.</p></li>
</ul>
<p>However, not all string values used as annotations can
be successfully turned into Python values by <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a>.
String values could theoretically contain any valid string,
and in practice there are valid use cases for type hints that
require annotating with string values that specifically
<em>can’t</em> be evaluated.  For example:</p>
<ul class="simple">
<li><p><span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0604"><strong>PEP 604</strong></a> union types using <code class="docutils literal notranslate"><span class="pre">|</span></code>, before support for this
was added to Python 3.10.</p></li>
<li><p>Definitions that aren’t needed at runtime, only imported
when <a class="reference internal" href="../library/typing.html#typing.TYPE_CHECKING" title="typing.TYPE_CHECKING"><code class="xref py py-const docutils literal notranslate"><span class="pre">typing.TYPE_CHECKING</span></code></a> is true.</p></li>
</ul>
<p>If <a class="reference internal" href="../library/functions.html#eval" title="eval"><code class="xref py py-func docutils literal notranslate"><span class="pre">eval()</span></code></a> attempts to evaluate such values, it will
fail and raise an exception.  So, when designing a library
API that works with annotations, it’s recommended to only
attempt to evaluate string values when explicitly requested
to by the caller.</p>
</div></blockquote>
</section>
<section id="best-practices-for-annotations-in-any-python-version">
<h2>Best Practices For <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> In Any Python Version<a class="headerlink" href="#best-practices-for-annotations-in-any-python-version" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div><ul class="simple">
<li><p>You should avoid assigning to the <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> member
of objects directly.  Let Python manage setting <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code>.</p></li>
<li><p>If you do assign directly to the <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> member
of an object, you should always set it to a <code class="docutils literal notranslate"><span class="pre">dict</span></code> object.</p></li>
<li><p>If you directly access the <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> member
of an object, you should ensure that it’s a
dictionary before attempting to examine its contents.</p></li>
<li><p>You should avoid modifying <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> dicts.</p></li>
<li><p>You should avoid deleting the <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> attribute
of an object.</p></li>
</ul>
</div></blockquote>
</section>
<section id="annotations-quirks">
<h2><code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> Quirks<a class="headerlink" href="#annotations-quirks" title="Permalink to this headline">¶</a></h2>
<blockquote>
<div><p>In all versions of Python 3, function
objects lazy-create an annotations dict if no annotations
are defined on that object.  You can delete the <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code>
attribute using <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">fn.__annotations__</span></code>, but if you then
access <code class="docutils literal notranslate"><span class="pre">fn.__annotations__</span></code> the object will create a new empty dict
that it will store and return as its annotations.  Deleting the
annotations on a function before it has lazily created its annotations
dict will throw an <code class="docutils literal notranslate"><span class="pre">AttributeError</span></code>; using <code class="docutils literal notranslate"><span class="pre">del</span> <span class="pre">fn.__annotations__</span></code>
twice in a row is guaranteed to always throw an <code class="docutils literal notranslate"><span class="pre">AttributeError</span></code>.</p>
<p>Everything in the above paragraph also applies to class and module
objects in Python 3.10 and newer.</p>
<p>In all versions of Python 3, you can set <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code>
on a function object to <code class="docutils literal notranslate"><span class="pre">None</span></code>.  However, subsequently
accessing the annotations on that object using <code class="docutils literal notranslate"><span class="pre">fn.__annotations__</span></code>
will lazy-create an empty dictionary as per the first paragraph of
this section.  This is <em>not</em> true of modules and classes, in any Python
version; those objects permit setting <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> to any
Python value, and will retain whatever value is set.</p>
<p>If Python stringizes your annotations for you
(using <code class="docutils literal notranslate"><span class="pre">from</span> <span class="pre">__future__</span> <span class="pre">import</span> <span class="pre">annotations</span></code>), and you
specify a string as an annotation, the string will
itself be quoted.  In effect the annotation is quoted
<em>twice.</em>  For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">from</span> <span class="nn">__future__</span> <span class="kn">import</span> <span class="n">annotations</span>
<span class="k">def</span> <span class="nf">foo</span><span class="p">(</span><span class="n">a</span><span class="p">:</span> <span class="s2">&quot;str&quot;</span><span class="p">):</span> <span class="k">pass</span>

<span class="nb">print</span><span class="p">(</span><span class="n">foo</span><span class="o">.</span><span class="vm">__annotations__</span><span class="p">)</span>
</pre></div>
</div>
<p>This prints <code class="docutils literal notranslate"><span class="pre">{'a':</span> <span class="pre">&quot;'str'&quot;}</span></code>.  This shouldn’t really be considered
a “quirk”; it’s mentioned here simply because it might be surprising.</p>
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<li><a class="reference internal" href="#">Annotations Best Practices</a><ul>
<li><a class="reference internal" href="#accessing-the-annotations-dict-of-an-object-in-python-3-10-and-newer">Accessing The Annotations Dict Of An Object In Python 3.10 And Newer</a></li>
<li><a class="reference internal" href="#accessing-the-annotations-dict-of-an-object-in-python-3-9-and-older">Accessing The Annotations Dict Of An Object In Python 3.9 And Older</a></li>
<li><a class="reference internal" href="#manually-un-stringizing-stringized-annotations">Manually Un-Stringizing Stringized Annotations</a></li>
<li><a class="reference internal" href="#best-practices-for-annotations-in-any-python-version">Best Practices For <code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> In Any Python Version</a></li>
<li><a class="reference internal" href="#annotations-quirks"><code class="docutils literal notranslate"><span class="pre">__annotations__</span></code> Quirks</a></li>
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