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<li><a class="reference internal" href="#">Supporting Cyclic Garbage Collection</a><ul>
<li><a class="reference internal" href="#controlling-the-garbage-collector-state">Controlling the Garbage Collector State</a></li>
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 <section id="supporting-cyclic-garbage-collection">
<h1>Supporting Cyclic Garbage Collection<a class="headerlink" href="#supporting-cyclic-garbage-collection" title="Permalink to this headline">¶</a></h1>
Python’s support for detecting and collecting garbage which involves circular
references requires support from object types which are “containers” for other
objects which may also be containers. Types which do not store references to
other objects, or which only store references to atomic types (such as numbers
or strings), do not need to provide any explicit support for garbage
collection.
To create a container type, the <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_flags" title="PyTypeObject.tp_flags"><code class="xref c c-member docutils literal notranslate">tp_flags</code></a> field of the type object must
include the <a class="reference internal" href="typeobj.html#Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref py py-const docutils literal notranslate">Py_TPFLAGS_HAVE_GC</code></a> and provide an implementation of the
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handler. If instances of the type are mutable, a
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_clear" title="PyTypeObject.tp_clear"><code class="xref c c-member docutils literal notranslate">tp_clear</code></a> implementation must also be provided.
<dl class="py data">
<dt class="sig sig-object py">
Py_TPFLAGS_HAVE_GC</dt>
<dd>Objects with a type with this flag set must conform with the rules
documented here. For convenience these objects will be referred to as
container objects.
</dd></dl>

Constructors for container types must conform to two rules:
<ol class="arabic simple">
<li>The memory for the object must be allocated using <a class="reference internal" href="#c.PyObject_GC_New" title="PyObject_GC_New"><code class="xref c c-func docutils literal notranslate">PyObject_GC_New()</code></a>
or <a class="reference internal" href="#c.PyObject_GC_NewVar" title="PyObject_GC_NewVar"><code class="xref c c-func docutils literal notranslate">PyObject_GC_NewVar()</code></a>.</li>
<li>Once all the fields which may contain references to other containers are
initialized, it must call <a class="reference internal" href="#c.PyObject_GC_Track" title="PyObject_GC_Track"><code class="xref c c-func docutils literal notranslate">PyObject_GC_Track()</code></a>.</li>
</ol>
Similarly, the deallocator for the object must conform to a similar pair of
rules:
<ol class="arabic">
<li>Before fields which refer to other containers are invalidated,
<a class="reference internal" href="#c.PyObject_GC_UnTrack" title="PyObject_GC_UnTrack"><code class="xref c c-func docutils literal notranslate">PyObject_GC_UnTrack()</code></a> must be called.</li>
<li>The object’s memory must be deallocated using <a class="reference internal" href="#c.PyObject_GC_Del" title="PyObject_GC_Del"><code class="xref c c-func docutils literal notranslate">PyObject_GC_Del()</code></a>.
<div class="admonition warning">
Warning
If a type adds the Py_TPFLAGS_HAVE_GC, then it must implement at least
a <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handler or explicitly use one
from its subclass or subclasses.
When calling <a class="reference internal" href="type.html#c.PyType_Ready" title="PyType_Ready"><code class="xref c c-func docutils literal notranslate">PyType_Ready()</code></a> or some of the APIs that indirectly
call it like <a class="reference internal" href="type.html#c.PyType_FromSpecWithBases" title="PyType_FromSpecWithBases"><code class="xref c c-func docutils literal notranslate">PyType_FromSpecWithBases()</code></a> or
<a class="reference internal" href="type.html#c.PyType_FromSpec" title="PyType_FromSpec"><code class="xref c c-func docutils literal notranslate">PyType_FromSpec()</code></a> the interpreter will automatically populate the
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_flags" title="PyTypeObject.tp_flags"><code class="xref c c-member docutils literal notranslate">tp_flags</code></a>, <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a>
and <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_clear" title="PyTypeObject.tp_clear"><code class="xref c c-member docutils literal notranslate">tp_clear</code></a> fields if the type inherits from a
class that implements the garbage collector protocol and the child class
does not include the <a class="reference internal" href="typeobj.html#Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref py py-const docutils literal notranslate">Py_TPFLAGS_HAVE_GC</code></a> flag.
</div>
</li>
</ol>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_New">
TYPE *PyObject_GC_New(TYPE, <a class="reference internal" href="type.html#c.PyTypeObject" title="PyTypeObject">PyTypeObject</a> *type)<a class="headerlink" href="#c.PyObject_GC_New" title="Permalink to this definition">¶</a> </dt>
<dd>Analogous to <a class="reference internal" href="allocation.html#c.PyObject_New" title="PyObject_New"><code class="xref c c-func docutils literal notranslate">PyObject_New()</code></a> but for container objects with the
<a class="reference internal" href="typeobj.html#Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref py py-const docutils literal notranslate">Py_TPFLAGS_HAVE_GC</code></a> flag set.
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_NewVar">
TYPE *PyObject_GC_NewVar(TYPE, <a class="reference internal" href="type.html#c.PyTypeObject" title="PyTypeObject">PyTypeObject</a> *type, <a class="reference internal" href="intro.html#c.Py_ssize_t" title="Py_ssize_t">Py_ssize_t</a> size)<a class="headerlink" href="#c.PyObject_GC_NewVar" title="Permalink to this definition">¶</a> </dt>
<dd>Analogous to <a class="reference internal" href="allocation.html#c.PyObject_NewVar" title="PyObject_NewVar"><code class="xref c c-func docutils literal notranslate">PyObject_NewVar()</code></a> but for container objects with the
<a class="reference internal" href="typeobj.html#Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref py py-const docutils literal notranslate">Py_TPFLAGS_HAVE_GC</code></a> flag set.
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_Resize">
TYPE *PyObject_GC_Resize(TYPE, <a class="reference internal" href="structures.html#c.PyVarObject" title="PyVarObject">PyVarObject</a> *op, <a class="reference internal" href="intro.html#c.Py_ssize_t" title="Py_ssize_t">Py_ssize_t</a> newsize)<a class="headerlink" href="#c.PyObject_GC_Resize" title="Permalink to this definition">¶</a> </dt>
<dd>Resize an object allocated by <a class="reference internal" href="allocation.html#c.PyObject_NewVar" title="PyObject_NewVar"><code class="xref c c-func docutils literal notranslate">PyObject_NewVar()</code></a>. Returns the
resized object or <code class="docutils literal notranslate">NULL</code> on failure. op must not be tracked by the collector yet.
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_Track">
void PyObject_GC_Track(<a class="reference internal" href="structures.html#c.PyObject" title="PyObject">PyObject</a> *op)<a class="headerlink" href="#c.PyObject_GC_Track" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a>.Adds the object op to the set of container objects tracked by the
collector. The collector can run at unexpected times so objects must be
valid while being tracked. This should be called once all the fields
followed by the <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handler become valid, usually near the
end of the constructor.
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_IS_GC">
int PyObject_IS_GC(<a class="reference internal" href="structures.html#c.PyObject" title="PyObject">PyObject</a> *obj)<a class="headerlink" href="#c.PyObject_IS_GC" title="Permalink to this definition">¶</a> </dt>
<dd>Returns non-zero if the object implements the garbage collector protocol,
otherwise returns 0.
The object cannot be tracked by the garbage collector if this function returns 0.
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_IsTracked">
int PyObject_GC_IsTracked(<a class="reference internal" href="structures.html#c.PyObject" title="PyObject">PyObject</a> *op)<a class="headerlink" href="#c.PyObject_GC_IsTracked" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a> since version 3.9.Returns 1 if the object type of op implements the GC protocol and op is being
currently tracked by the garbage collector and 0 otherwise.
This is analogous to the Python function <a class="reference internal" href="../library/gc.html#gc.is_tracked" title="gc.is_tracked"><code class="xref py py-func docutils literal notranslate">gc.is_tracked()</code></a>.
<div class="versionadded">
New in version 3.9.
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_IsFinalized">
int PyObject_GC_IsFinalized(<a class="reference internal" href="structures.html#c.PyObject" title="PyObject">PyObject</a> *op)<a class="headerlink" href="#c.PyObject_GC_IsFinalized" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a> since version 3.9.Returns 1 if the object type of op implements the GC protocol and op has been
already finalized by the garbage collector and 0 otherwise.
This is analogous to the Python function <a class="reference internal" href="../library/gc.html#gc.is_finalized" title="gc.is_finalized"><code class="xref py py-func docutils literal notranslate">gc.is_finalized()</code></a>.
<div class="versionadded">
New in version 3.9.
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_Del">
void PyObject_GC_Del(void *op)<a class="headerlink" href="#c.PyObject_GC_Del" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a>.Releases memory allocated to an object using <a class="reference internal" href="#c.PyObject_GC_New" title="PyObject_GC_New"><code class="xref c c-func docutils literal notranslate">PyObject_GC_New()</code></a> or
<a class="reference internal" href="#c.PyObject_GC_NewVar" title="PyObject_GC_NewVar"><code class="xref c c-func docutils literal notranslate">PyObject_GC_NewVar()</code></a>.
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_UnTrack">
void PyObject_GC_UnTrack(void *op)<a class="headerlink" href="#c.PyObject_GC_UnTrack" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a>.Remove the object op from the set of container objects tracked by the
collector. Note that <a class="reference internal" href="#c.PyObject_GC_Track" title="PyObject_GC_Track"><code class="xref c c-func docutils literal notranslate">PyObject_GC_Track()</code></a> can be called again on
this object to add it back to the set of tracked objects. The deallocator
(<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_dealloc" title="PyTypeObject.tp_dealloc"><code class="xref c c-member docutils literal notranslate">tp_dealloc</code></a> handler) should call this for the object before any of
the fields used by the <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handler become invalid.
</dd></dl>

<div class="versionchanged">
Changed in version 3.8: The <code class="xref c c-func docutils literal notranslate">_PyObject_GC_TRACK()</code> and <code class="xref c c-func docutils literal notranslate">_PyObject_GC_UNTRACK()</code> macros
have been removed from the public C API.
</div>
The <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handler accepts a function parameter of this type:
<dl class="c type">
<dt class="sig sig-object c" id="c.visitproc">
typedef int (*visitproc)(<a class="reference internal" href="structures.html#c.PyObject" title="PyObject">PyObject</a> *object, void *arg)<a class="headerlink" href="#c.visitproc" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a>.Type of the visitor function passed to the <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handler.
The function should be called with an object to traverse as object and
the third parameter to the <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handler as arg. The
Python core uses several visitor functions to implement cyclic garbage
detection; it’s not expected that users will need to write their own
visitor functions.
</dd></dl>

The <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handler must have the following type:
<dl class="c type">
<dt class="sig sig-object c" id="c.traverseproc">
typedef int (*traverseproc)(<a class="reference internal" href="structures.html#c.PyObject" title="PyObject">PyObject</a> *self, <a class="reference internal" href="#c.visitproc" title="visitproc">visitproc</a> visit, void *arg)<a class="headerlink" href="#c.traverseproc" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a>.Traversal function for a container object. Implementations must call the
visit function for each object directly contained by self, with the
parameters to visit being the contained object and the arg value passed
to the handler. The visit function must not be called with a <code class="docutils literal notranslate">NULL</code>
object argument. If visit returns a non-zero value that value should be
returned immediately.
</dd></dl>

To simplify writing <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handlers, a <a class="reference internal" href="#c.Py_VISIT" title="Py_VISIT"><code class="xref c c-func docutils literal notranslate">Py_VISIT()</code></a> macro is
provided. In order to use this macro, the <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> implementation
must name its arguments exactly visit and arg:
<dl class="c function">
<dt class="sig sig-object c" id="c.Py_VISIT">
void Py_VISIT(<a class="reference internal" href="structures.html#c.PyObject" title="PyObject">PyObject</a> *o)<a class="headerlink" href="#c.Py_VISIT" title="Permalink to this definition">¶</a> </dt>
<dd>If o is not <code class="docutils literal notranslate">NULL</code>, call the visit callback, with arguments o
and arg. If visit returns a non-zero value, then return it.
Using this macro, <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_traverse" title="PyTypeObject.tp_traverse"><code class="xref c c-member docutils literal notranslate">tp_traverse</code></a> handlers
look like:
<div class="highlight-c notranslate"><div class="highlight"><pre>static int
my_traverse(Noddy *self, visitproc visit, void *arg)
{
 Py_VISIT(self-&gt;foo);
 Py_VISIT(self-&gt;bar);
 return 0;
}
</pre></div>
</div>
</dd></dl>

The <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_clear" title="PyTypeObject.tp_clear"><code class="xref c c-member docutils literal notranslate">tp_clear</code></a> handler must be of the <a class="reference internal" href="#c.inquiry" title="inquiry"><code class="xref c c-type docutils literal notranslate">inquiry</code></a> type, or <code class="docutils literal notranslate">NULL</code>
if the object is immutable.
<dl class="c type">
<dt class="sig sig-object c" id="c.inquiry">
typedef int (*inquiry)(<a class="reference internal" href="structures.html#c.PyObject" title="PyObject">PyObject</a> *self)<a class="headerlink" href="#c.inquiry" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a>.Drop references that may have created reference cycles. Immutable objects
do not have to define this method since they can never directly create
reference cycles. Note that the object must still be valid after calling
this method (don’t just call <a class="reference internal" href="refcounting.html#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate">Py_DECREF()</code></a> on a reference). The
collector will call this method if it detects that this object is involved
in a reference cycle.
</dd></dl>

<section id="controlling-the-garbage-collector-state">
<h2>Controlling the Garbage Collector State<a class="headerlink" href="#controlling-the-garbage-collector-state" title="Permalink to this headline">¶</a></h2>
The C-API provides the following functions for controlling
garbage collection runs.
<dl class="c function">
<dt class="sig sig-object c" id="c.PyGC_Collect">
<a class="reference internal" href="intro.html#c.Py_ssize_t" title="Py_ssize_t">Py_ssize_t</a> PyGC_Collect(void)<a class="headerlink" href="#c.PyGC_Collect" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a>.Perform a full garbage collection, if the garbage collector is enabled.
(Note that <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> runs it unconditionally.)
Returns the number of collected + unreachable objects which cannot
be collected.
If the garbage collector is disabled or already collecting,
returns <code class="docutils literal notranslate">0</code> immediately.
Errors during garbage collection are passed to <a class="reference internal" href="../library/sys.html#sys.unraisablehook" title="sys.unraisablehook"><code class="xref py py-data docutils literal notranslate">sys.unraisablehook</code></a>.
This function does not raise exceptions.
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyGC_Enable">
int PyGC_Enable(void)<a class="headerlink" href="#c.PyGC_Enable" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a> since version 3.10.Enable the garbage collector: similar to <a class="reference internal" href="../library/gc.html#gc.enable" title="gc.enable"><code class="xref py py-func docutils literal notranslate">gc.enable()</code></a>.
Returns the previous state, 0 for disabled and 1 for enabled.
<div class="versionadded">
New in version 3.10.
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyGC_Disable">
int PyGC_Disable(void)<a class="headerlink" href="#c.PyGC_Disable" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a> since version 3.10.Disable the garbage collector: similar to <a class="reference internal" href="../library/gc.html#gc.disable" title="gc.disable"><code class="xref py py-func docutils literal notranslate">gc.disable()</code></a>.
Returns the previous state, 0 for disabled and 1 for enabled.
<div class="versionadded">
New in version 3.10.
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.PyGC_IsEnabled">
int PyGC_IsEnabled(void)<a class="headerlink" href="#c.PyGC_IsEnabled" title="Permalink to this definition">¶</a> </dt>
<dd> Part of the <a class="reference internal" href="stable.html#stable">Stable ABI</a> since version 3.10.Query the state of the garbage collector: similar to <a class="reference internal" href="../library/gc.html#gc.isenabled" title="gc.isenabled"><code class="xref py py-func docutils literal notranslate">gc.isenabled()</code></a>.
Returns the current state, 0 for disabled and 1 for enabled.
<div class="versionadded">
New in version 3.10.
</div>
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