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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>
<li><a class="reference internal" href="#querying-garbage-collector-state">Querying Garbage Collector State</a></li>
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<section id="supporting-cyclic-garbage-collection">
<span id="supporting-cycle-detection"></span><h1>Supporting Cyclic Garbage Collection<a class="headerlink" href="#supporting-cyclic-garbage-collection" title="Link to this heading"></a></h1>
<p>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.</p>
<p>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"><span class="pre">tp_flags</span></code></a> field of the type object must
include the <a class="reference internal" href="typeobj.html#c.Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_TPFLAGS_HAVE_GC</span></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"><span class="pre">tp_traverse</span></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"><span class="pre">tp_clear</span></code></a> implementation must also be provided.</p>
<dl class="simple">
<dt><a class="reference internal" href="typeobj.html#c.Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_TPFLAGS_HAVE_GC</span></code></a></dt><dd><p>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.</p>
</dd>
</dl>
<p>Constructors for container types must conform to two rules:</p>
<ol class="arabic simple">
<li><p>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-macro docutils literal notranslate"><span class="pre">PyObject_GC_New</span></code></a>
or <a class="reference internal" href="#c.PyObject_GC_NewVar" title="PyObject_GC_NewVar"><code class="xref c c-macro docutils literal notranslate"><span class="pre">PyObject_GC_NewVar</span></code></a>.</p></li>
<li><p>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"><span class="pre">PyObject_GC_Track()</span></code></a>.</p></li>
</ol>
<p>Similarly, the deallocator for the object must conform to a similar pair of
rules:</p>
<ol class="arabic">
<li><p>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"><span class="pre">PyObject_GC_UnTrack()</span></code></a> must be called.</p></li>
<li><p>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"><span class="pre">PyObject_GC_Del()</span></code></a>.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>If a type adds the Py_TPFLAGS_HAVE_GC, then it <em>must</em> 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"><span class="pre">tp_traverse</span></code></a> handler or explicitly use one
from its subclass or subclasses.</p>
<p>When calling <a class="reference internal" href="type.html#c.PyType_Ready" title="PyType_Ready"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_Ready()</span></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"><span class="pre">PyType_FromSpecWithBases()</span></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"><span class="pre">PyType_FromSpec()</span></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"><span class="pre">tp_flags</span></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"><span class="pre">tp_traverse</span></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"><span class="pre">tp_clear</span></code></a> fields if the type inherits from a
class that implements the garbage collector protocol and the child class
does <em>not</em> include the <a class="reference internal" href="typeobj.html#c.Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_TPFLAGS_HAVE_GC</span></code></a> flag.</p>
</div>
</li>
</ol>
<dl class="c macro">
<dt class="sig sig-object c" id="c.PyObject_GC_New">
<span class="sig-name descname"><span class="n"><span class="pre">PyObject_GC_New</span></span></span><span class="sig-paren">(</span><span class="n"><span class="pre">TYPE</span></span>, <span class="n"><span class="pre">typeobj</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GC_New" title="Link to this definition"></a><br /></dt>
<dd><p>Analogous to <a class="reference internal" href="allocation.html#c.PyObject_New" title="PyObject_New"><code class="xref c c-macro docutils literal notranslate"><span class="pre">PyObject_New</span></code></a> but for container objects with the
<a class="reference internal" href="typeobj.html#c.Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_TPFLAGS_HAVE_GC</span></code></a> flag set.</p>
</dd></dl>
<dl class="c macro">
<dt class="sig sig-object c" id="c.PyObject_GC_NewVar">
<span class="sig-name descname"><span class="n"><span class="pre">PyObject_GC_NewVar</span></span></span><span class="sig-paren">(</span><span class="n"><span class="pre">TYPE</span></span>, <span class="n"><span class="pre">typeobj</span></span>, <span class="n"><span class="pre">size</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GC_NewVar" title="Link to this definition"></a><br /></dt>
<dd><p>Analogous to <a class="reference internal" href="allocation.html#c.PyObject_NewVar" title="PyObject_NewVar"><code class="xref c c-macro docutils literal notranslate"><span class="pre">PyObject_NewVar</span></code></a> but for container objects with the
<a class="reference internal" href="typeobj.html#c.Py_TPFLAGS_HAVE_GC" title="Py_TPFLAGS_HAVE_GC"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_TPFLAGS_HAVE_GC</span></code></a> flag set.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyUnstable_Object_GC_NewWithExtraData">
<a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyUnstable_Object_GC_NewWithExtraData</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="type.html#c.PyTypeObject" title="PyTypeObject"><span class="n"><span class="pre">PyTypeObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">type</span></span>, <span class="n"><span class="pre">size_t</span></span><span class="w"> </span><span class="n"><span class="pre">extra_size</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyUnstable_Object_GC_NewWithExtraData" title="Link to this definition"></a><br /></dt>
<dd><div class="unstable-c-api warning admonition">
<em>This is <a class="reference internal" href="stable.html#unstable-c-api"><span class="std std-ref">Unstable API</span></a>. It may change without warning in minor releases.</em></div>
<p>Analogous to <a class="reference internal" href="#c.PyObject_GC_New" title="PyObject_GC_New"><code class="xref c c-macro docutils literal notranslate"><span class="pre">PyObject_GC_New</span></code></a> but allocates <em>extra_size</em>
bytes at the end of the object (at offset
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_basicsize" title="PyTypeObject.tp_basicsize"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_basicsize</span></code></a>).
The allocated memory is initialized to zeros,
except for the <a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">Python</span> <span class="pre">object</span> <span class="pre">header</span></code></a>.</p>
<p>The extra data will be deallocated with the object, but otherwise it is
not managed by Python.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>The function is marked as unstable because the final mechanism
for reserving extra data after an instance is not yet decided.
For allocating a variable number of fields, prefer using
<a class="reference internal" href="structures.html#c.PyVarObject" title="PyVarObject"><code class="xref c c-type docutils literal notranslate"><span class="pre">PyVarObject</span></code></a> and <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_itemsize" title="PyTypeObject.tp_itemsize"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_itemsize</span></code></a>
instead.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.12.</span></p>
</div>
</dd></dl>
<dl class="c macro">
<dt class="sig sig-object c" id="c.PyObject_GC_Resize">
<span class="sig-name descname"><span class="n"><span class="pre">PyObject_GC_Resize</span></span></span><span class="sig-paren">(</span><span class="n"><span class="pre">TYPE</span></span>, <span class="n"><span class="pre">op</span></span>, <span class="n"><span class="pre">newsize</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GC_Resize" title="Link to this definition"></a><br /></dt>
<dd><p>Resize an object allocated by <a class="reference internal" href="allocation.html#c.PyObject_NewVar" title="PyObject_NewVar"><code class="xref c c-macro docutils literal notranslate"><span class="pre">PyObject_NewVar</span></code></a>.
Returns the resized object of type <code class="docutils literal notranslate"><span class="pre">TYPE*</span></code> (refers to any C type)
or <code class="docutils literal notranslate"><span class="pre">NULL</span></code> on failure.</p>
<p><em>op</em> must be of type <span class="c-expr sig sig-inline c"><a class="reference internal" href="structures.html#c.PyVarObject" title="PyVarObject"><span class="n">PyVarObject</span></a><span class="p">*</span></span>
and must not be tracked by the collector yet.
<em>newsize</em> must be of type <a class="reference internal" href="intro.html#c.Py_ssize_t" title="Py_ssize_t"><code class="xref c c-type docutils literal notranslate"><span class="pre">Py_ssize_t</span></code></a>.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_Track">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_GC_Track</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">op</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GC_Track" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>Adds the object <em>op</em> 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"><span class="pre">tp_traverse</span></code></a> handler become valid, usually near the
end of the constructor.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_IS_GC">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_IS_GC</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">obj</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_IS_GC" title="Link to this definition"></a><br /></dt>
<dd><p>Returns non-zero if the object implements the garbage collector protocol,
otherwise returns 0.</p>
<p>The object cannot be tracked by the garbage collector if this function returns 0.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_IsTracked">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_GC_IsTracked</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">op</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GC_IsTracked" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a> since version 3.9.</em><p>Returns 1 if the object type of <em>op</em> implements the GC protocol and <em>op</em> is being
currently tracked by the garbage collector and 0 otherwise.</p>
<p>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"><span class="pre">gc.is_tracked()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.9.</span></p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_IsFinalized">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_GC_IsFinalized</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">op</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GC_IsFinalized" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a> since version 3.9.</em><p>Returns 1 if the object type of <em>op</em> implements the GC protocol and <em>op</em> has been
already finalized by the garbage collector and 0 otherwise.</p>
<p>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"><span class="pre">gc.is_finalized()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.9.</span></p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_Del">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_GC_Del</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">op</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GC_Del" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>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-macro docutils literal notranslate"><span class="pre">PyObject_GC_New</span></code></a> or
<a class="reference internal" href="#c.PyObject_GC_NewVar" title="PyObject_GC_NewVar"><code class="xref c c-macro docutils literal notranslate"><span class="pre">PyObject_GC_NewVar</span></code></a>.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GC_UnTrack">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_GC_UnTrack</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">op</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GC_UnTrack" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>Remove the object <em>op</em> 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"><span class="pre">PyObject_GC_Track()</span></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"><span class="pre">tp_dealloc</span></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"><span class="pre">tp_traverse</span></code></a> handler become invalid.</p>
</dd></dl>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.8: </span>The <code class="xref c c-func docutils literal notranslate"><span class="pre">_PyObject_GC_TRACK()</span></code> and <code class="xref c c-func docutils literal notranslate"><span class="pre">_PyObject_GC_UNTRACK()</span></code> macros
have been removed from the public C API.</p>
</div>
<p>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"><span class="pre">tp_traverse</span></code></a> handler accepts a function parameter of this type:</p>
<dl class="c type">
<dt class="sig sig-object c" id="c.visitproc">
<span class="k"><span class="pre">typedef</span></span><span class="w"> </span><span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="p"><span class="pre">(</span></span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">visitproc</span></span></span><span class="p"><span class="pre">)</span></span><span class="p"><span class="pre">(</span></span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">object</span></span><span class="p"><span class="pre">,</span></span><span class="w"> </span><span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">arg</span></span><span class="p"><span class="pre">)</span></span><a class="headerlink" href="#c.visitproc" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>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"><span class="pre">tp_traverse</span></code></a> handler.
The function should be called with an object to traverse as <em>object</em> 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"><span class="pre">tp_traverse</span></code></a> handler as <em>arg</em>. 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.</p>
</dd></dl>
<p>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"><span class="pre">tp_traverse</span></code></a> handler must have the following type:</p>
<dl class="c type">
<dt class="sig sig-object c" id="c.traverseproc">
<span class="k"><span class="pre">typedef</span></span><span class="w"> </span><span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="p"><span class="pre">(</span></span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">traverseproc</span></span></span><span class="p"><span class="pre">)</span></span><span class="p"><span class="pre">(</span></span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">self</span></span><span class="p"><span class="pre">,</span></span><span class="w"> </span><a class="reference internal" href="#c.visitproc" title="visitproc"><span class="n"><span class="pre">visitproc</span></span></a><span class="w"> </span><span class="n"><span class="pre">visit</span></span><span class="p"><span class="pre">,</span></span><span class="w"> </span><span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">arg</span></span><span class="p"><span class="pre">)</span></span><a class="headerlink" href="#c.traverseproc" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>Traversal function for a container object. Implementations must call the
<em>visit</em> function for each object directly contained by <em>self</em>, with the
parameters to <em>visit</em> being the contained object and the <em>arg</em> value passed
to the handler. The <em>visit</em> function must not be called with a <code class="docutils literal notranslate"><span class="pre">NULL</span></code>
object argument. If <em>visit</em> returns a non-zero value that value should be
returned immediately.</p>
</dd></dl>
<p>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"><span class="pre">tp_traverse</span></code></a> handlers, a <a class="reference internal" href="#c.Py_VISIT" title="Py_VISIT"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_VISIT()</span></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"><span class="pre">tp_traverse</span></code></a> implementation
must name its arguments exactly <em>visit</em> and <em>arg</em>:</p>
<dl class="c function">
<dt class="sig sig-object c" id="c.Py_VISIT">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">Py_VISIT</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">o</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.Py_VISIT" title="Link to this definition"></a><br /></dt>
<dd><p>If <em>o</em> is not <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, call the <em>visit</em> callback, with arguments <em>o</em>
and <em>arg</em>. If <em>visit</em> 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"><span class="pre">tp_traverse</span></code></a> handlers
look like:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="k">static</span><span class="w"> </span><span class="kt">int</span>
<span class="nf">my_traverse</span><span class="p">(</span><span class="n">Noddy</span><span class="w"> </span><span class="o">*</span><span class="n">self</span><span class="p">,</span><span class="w"> </span><span class="n">visitproc</span><span class="w"> </span><span class="n">visit</span><span class="p">,</span><span class="w"> </span><span class="kt">void</span><span class="w"> </span><span class="o">*</span><span class="n">arg</span><span class="p">)</span>
<span class="p">{</span>
<span class="w"> </span><span class="n">Py_VISIT</span><span class="p">(</span><span class="n">self</span><span class="o">-&gt;</span><span class="n">foo</span><span class="p">);</span>
<span class="w"> </span><span class="n">Py_VISIT</span><span class="p">(</span><span class="n">self</span><span class="o">-&gt;</span><span class="n">bar</span><span class="p">);</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
</dd></dl>
<p>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"><span class="pre">tp_clear</span></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"><span class="pre">inquiry</span></code></a> type, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code>
if the object is immutable.</p>
<dl class="c type">
<dt class="sig sig-object c" id="c.inquiry">
<span class="k"><span class="pre">typedef</span></span><span class="w"> </span><span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="p"><span class="pre">(</span></span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">inquiry</span></span></span><span class="p"><span class="pre">)</span></span><span class="p"><span class="pre">(</span></span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">self</span></span><span class="p"><span class="pre">)</span></span><a class="headerlink" href="#c.inquiry" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>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"><span class="pre">Py_DECREF()</span></code></a> on a reference). The
collector will call this method if it detects that this object is involved
in a reference cycle.</p>
</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="Link to this heading"></a></h2>
<p>The C-API provides the following functions for controlling
garbage collection runs.</p>
<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"><span class="n"><span class="pre">Py_ssize_t</span></span></a><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyGC_Collect</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyGC_Collect" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>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"><span class="pre">gc.collect()</span></code></a> runs it unconditionally.)</p>
<p>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"><span class="pre">0</span></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"><span class="pre">sys.unraisablehook</span></code></a>.
This function does not raise exceptions.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyGC_Enable">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyGC_Enable</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyGC_Enable" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a> since version 3.10.</em><p>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"><span class="pre">gc.enable()</span></code></a>.
Returns the previous state, 0 for disabled and 1 for enabled.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.10.</span></p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyGC_Disable">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyGC_Disable</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyGC_Disable" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a> since version 3.10.</em><p>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"><span class="pre">gc.disable()</span></code></a>.
Returns the previous state, 0 for disabled and 1 for enabled.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.10.</span></p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyGC_IsEnabled">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyGC_IsEnabled</span></span></span><span class="sig-paren">(</span><span class="kt"><span class="pre">void</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyGC_IsEnabled" title="Link to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a> since version 3.10.</em><p>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"><span class="pre">gc.isenabled()</span></code></a>.
Returns the current state, 0 for disabled and 1 for enabled.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.10.</span></p>
</div>
</dd></dl>
</section>
<section id="querying-garbage-collector-state">
<h2>Querying Garbage Collector State<a class="headerlink" href="#querying-garbage-collector-state" title="Link to this heading"></a></h2>
<p>The C-API provides the following interface for querying information about
the garbage collector.</p>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyUnstable_GC_VisitObjects">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyUnstable_GC_VisitObjects</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="#c.gcvisitobjects_t" title="gcvisitobjects_t"><span class="n"><span class="pre">gcvisitobjects_t</span></span></a><span class="w"> </span><span class="n"><span class="pre">callback</span></span>, <span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">arg</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyUnstable_GC_VisitObjects" title="Link to this definition"></a><br /></dt>
<dd><div class="unstable-c-api warning admonition">
<em>This is <a class="reference internal" href="stable.html#unstable-c-api"><span class="std std-ref">Unstable API</span></a>. It may change without warning in minor releases.</em></div>
<p>Run supplied <em>callback</em> on all live GC-capable objects. <em>arg</em> is passed through to
all invocations of <em>callback</em>.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>If new objects are (de)allocated by the callback it is undefined if they
will be visited.</p>
<p>Garbage collection is disabled during operation. Explicitly running a collection
in the callback may lead to undefined behaviour e.g. visiting the same objects
multiple times or not at all.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.12.</span></p>
</div>
</dd></dl>
<dl class="c type">
<dt class="sig sig-object c" id="c.gcvisitobjects_t">
<span class="k"><span class="pre">typedef</span></span><span class="w"> </span><span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="p"><span class="pre">(</span></span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">gcvisitobjects_t</span></span></span><span class="p"><span class="pre">)</span></span><span class="p"><span class="pre">(</span></span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">object</span></span><span class="p"><span class="pre">,</span></span><span class="w"> </span><span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">arg</span></span><span class="p"><span class="pre">)</span></span><a class="headerlink" href="#c.gcvisitobjects_t" title="Link to this definition"></a><br /></dt>
<dd><p>Type of the visitor function to be passed to <a class="reference internal" href="#c.PyUnstable_GC_VisitObjects" title="PyUnstable_GC_VisitObjects"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyUnstable_GC_VisitObjects()</span></code></a>.
<em>arg</em> is the same as the <em>arg</em> passed to <code class="docutils literal notranslate"><span class="pre">PyUnstable_GC_VisitObjects</span></code>.
Return <code class="docutils literal notranslate"><span class="pre">0</span></code> to continue iteration, return <code class="docutils literal notranslate"><span class="pre">1</span></code> to stop iteration. Other return
values are reserved for now so behavior on returning anything else is undefined.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.12.</span></p>
</div>
</dd></dl>
</section>
</section>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<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>
<li><a class="reference internal" href="#querying-garbage-collector-state">Querying Garbage Collector State</a></li>
</ul>
</li>
</ul>
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