| Server IP : 121.121.20.254 / Your IP : 216.73.216.165 Web Server : Microsoft-IIS/10.0 System : Windows NT WEB-SERVER 10.0 build 20348 (Windows Server 2022) AMD64 User : IUSR ( 0) PHP Version : 8.3.28 Disable Function : NONE MySQL : ON | cURL : ON | WGET : OFF | Perl : OFF | Python : OFF | Sudo : OFF | Pkexec : OFF Directory : C:/Python315/Doc/html/c-api/ |
Upload File : |
<!DOCTYPE html>
<html lang="en" data-content_root="../">
<head>
<meta charset="utf-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" /><meta name="viewport" content="width=device-width, initial-scale=1" />
<meta property="og:title" content="Supporting Cyclic Garbage Collection" />
<meta property="og:type" content="website" />
<meta property="og:url" content="https://docs.python.org/3/c-api/gcsupport.html" />
<meta property="og:site_name" content="Python documentation" />
<meta property="og:description" content="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. Ty..." />
<meta property="og:image" content="_static/og-image.png" />
<meta property="og:image:alt" content="Python documentation" />
<meta name="description" content="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. Ty..." />
<meta name="theme-color" content="#3776ab">
<meta property="og:image:width" content="200">
<meta property="og:image:height" content="200">
<title>Supporting Cyclic Garbage Collection — Python 3.15.0rc1 documentation</title><meta name="viewport" content="width=device-width, initial-scale=1.0">
<link rel="stylesheet" type="text/css" href="../_static/pygments.css?v=b86133f3" />
<link rel="stylesheet" type="text/css" href="../_static/classic.css?v=234b1a7c" />
<link rel="stylesheet" type="text/css" href="../_static/pydoctheme.css?v=4365c8fe" />
<link id="pygments_dark_css" media="(prefers-color-scheme: dark)" rel="stylesheet" type="text/css" href="../_static/pygments_dark.css?v=0fc419ee" />
<script src="../_static/documentation_options.js?v=791e38b1"></script>
<script src="../_static/doctools.js?v=9bcbadda"></script>
<script src="../_static/sphinx_highlight.js?v=dc90522c"></script>
<script src="../_static/sidebar.js"></script>
<link rel="search" type="application/opensearchdescription+xml"
title="Search within Python 3.15.0rc1 documentation"
href="../_static/opensearch.xml"/>
<link rel="author" title="About these documents" href="../about.html" />
<link rel="index" title="Index" href="../genindex.html" />
<link rel="search" title="Search" href="../search.html" />
<link rel="copyright" title="Copyright" href="../copyright.html" />
<link rel="next" title="API and ABI Versioning" href="apiabiversion.html" />
<link rel="prev" title="Type Object Structures" href="typeobj.html" />
<link rel="canonical" href="https://docs.python.org/3/c-api/gcsupport.html">
<style>
@media only screen {
table.full-width-table {
width: 100%;
}
}
</style>
<link rel="stylesheet" href="../_static/pydoctheme_dark.css" media="(prefers-color-scheme: dark)" id="pydoctheme_dark_css">
<link rel="shortcut icon" type="image/png" href="../_static/py.svg">
<script type="text/javascript" src="../_static/copybutton.js"></script>
<script type="text/javascript" src="../_static/menu.js"></script>
<script type="text/javascript" src="../_static/search-focus.js"></script>
<script type="text/javascript" src="../_static/themetoggle.js"></script>
<script type="text/javascript" src="../_static/rtd_switcher.js"></script>
<meta name="readthedocs-addons-api-version" content="1">
</head>
<body>
<div class="mobile-nav">
<input type="checkbox" id="menuToggler" class="toggler__input" aria-controls="navigation"
aria-pressed="false" aria-expanded="false" role="button" aria-label="Menu">
<nav class="nav-content" role="navigation">
<label for="menuToggler" class="toggler__label">
<span></span>
</label>
<span class="nav-items-wrapper">
<a href="https://www.python.org/" class="nav-logo">
<img src="../_static/py.svg" alt="Python logo">
</a>
<span class="version_switcher_placeholder"></span>
<form role="search" class="search" action="../search.html" method="get">
<svg xmlns="http://www.w3.org/2000/svg" width="20" height="20" viewBox="0 0 24 24" class="search-icon">
<path fill-rule="nonzero" fill="currentColor" d="M15.5 14h-.79l-.28-.27a6.5 6.5 0 001.48-5.34c-.47-2.78-2.79-5-5.59-5.34a6.505 6.505 0 00-7.27 7.27c.34 2.8 2.56 5.12 5.34 5.59a6.5 6.5 0 005.34-1.48l.27.28v.79l4.25 4.25c.41.41 1.08.41 1.49 0 .41-.41.41-1.08 0-1.49L15.5 14zm-6 0C7.01 14 5 11.99 5 9.5S7.01 5 9.5 5 14 7.01 14 9.5 11.99 14 9.5 14z"></path>
</svg>
<input placeholder="Quick search" aria-label="Quick search" type="search" name="q">
<input type="submit" value="Go">
</form>
</span>
</nav>
<div class="menu-wrapper">
<nav class="menu" role="navigation" aria-label="main navigation">
<div class="language_switcher_placeholder"></div>
<label class="theme-selector-label">
Theme
<select class="theme-selector" oninput="activateTheme(this.value)">
<option value="auto" selected>Auto</option>
<option value="light">Light</option>
<option value="dark">Dark</option>
</select>
</label>
<div>
<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="#traversal">Traversal</a><ul>
<li><a class="reference internal" href="#traversal-safe-functions">Traversal-safe functions</a></li>
<li><a class="reference internal" href="#duringgc-functions">“DuringGC” functions</a></li>
</ul>
</li>
<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>
</div>
<div>
<h4>Previous topic</h4>
<p class="topless"><a href="typeobj.html"
title="previous chapter">Type Object Structures</a></p>
</div>
<div>
<h4>Next topic</h4>
<p class="topless"><a href="apiabiversion.html"
title="next chapter">API and ABI Versioning</a></p>
</div>
<script>
document.addEventListener('DOMContentLoaded', () => {
const title = document.querySelector('meta[property="og:title"]').content;
const elements = document.querySelectorAll('.improvepage');
const pageurl = window.location.href.split('?')[0];
elements.forEach(element => {
const url = new URL(element.href.split('?')[0].replace("-nojs", ""));
url.searchParams.set('pagetitle', title);
url.searchParams.set('pageurl', pageurl);
url.searchParams.set('pagesource', "c-api/gcsupport.rst");
element.href = url.toString();
});
});
</script>
<div role="note" aria-label="source link">
<h3>This page</h3>
<ul class="this-page-menu">
<li><a href="../bugs.html">Report a bug</a></li>
<li><a class="improvepage" href="../improve-page-nojs.html">Improve this page</a></li>
<li>
<a href="https://github.com/python/cpython/blob/main/Doc/c-api/gcsupport.rst?plain=1"
rel="nofollow">Show source
</a>
</li>
</ul>
</div>
</nav>
</div>
</div>
<div class="related" role="navigation" aria-label="Related">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="../genindex.html" title="General Index"
accesskey="I">index</a></li>
<li class="right" >
<a href="../py-modindex.html" title="Python Module Index"
>modules</a> |</li>
<li class="right" >
<a href="apiabiversion.html" title="API and ABI Versioning"
accesskey="N">next</a> |</li>
<li class="right" >
<a href="typeobj.html" title="Type Object Structures"
accesskey="P">previous</a> |</li>
<li><img src="../_static/py.svg" alt="Python logo" style="vertical-align: middle; margin-top: -1px"></li>
<li><a href="https://www.python.org/">Python</a> »</li>
<li class="switchers">
<div class="language_switcher_placeholder"></div>
<div class="version_switcher_placeholder"></div>
</li>
<li>
</li>
<li id="cpython-language-and-version">
<a href="../index.html">3.15.0rc1 Documentation</a> »
</li>
<li class="nav-item nav-item-1"><a href="index.html" >Python/C API reference manual</a> »</li>
<li class="nav-item nav-item-2"><a href="objimpl.html" accesskey="U">Object Implementation Support</a> »</li>
<li class="nav-item nav-item-this"><a href="">Supporting Cyclic Garbage Collection</a></li>
<li class="right">
<div class="inline-search" role="search">
<form class="inline-search" action="../search.html" method="get">
<input placeholder="Quick search" aria-label="Quick search" type="search" name="q" id="search-box">
<input type="submit" value="Go">
</form>
</div>
|
</li>
<li class="right">
<label class="theme-selector-label">
Theme
<select class="theme-selector" oninput="activateTheme(this.value)">
<option value="auto" selected>Auto</option>
<option value="light">Light</option>
<option value="dark">Dark</option>
</select>
</label> |</li>
</ul>
</div>
<div class="document">
<div class="documentwrapper">
<div class="bodywrapper">
<div class="body" role="main">
<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>
<p>Do not call this directly to allocate memory for an object; call the type’s
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_alloc" title="PyTypeObject.tp_alloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_alloc</span></code></a> slot instead.</p>
<p>When populating a type’s <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_alloc" title="PyTypeObject.tp_alloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_alloc</span></code></a> slot,
<a class="reference internal" href="type.html#c.PyType_GenericAlloc" title="PyType_GenericAlloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GenericAlloc()</span></code></a> is preferred over a custom function that
simply calls this macro.</p>
<p>Memory allocated by this macro must be freed with
<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> (usually called via the object’s
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_free" title="PyTypeObject.tp_free"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_free</span></code></a> slot).</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<ul class="simple">
<li><p><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></li>
<li><p><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></p></li>
<li><p><a class="reference internal" href="type.html#c.PyType_GenericAlloc" title="PyType_GenericAlloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GenericAlloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_alloc" title="PyTypeObject.tp_alloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_alloc</span></code></a></p></li>
</ul>
</div>
</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>
<p>Do not call this directly to allocate memory for an object; call the type’s
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_alloc" title="PyTypeObject.tp_alloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_alloc</span></code></a> slot instead.</p>
<p>When populating a type’s <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_alloc" title="PyTypeObject.tp_alloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_alloc</span></code></a> slot,
<a class="reference internal" href="type.html#c.PyType_GenericAlloc" title="PyType_GenericAlloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GenericAlloc()</span></code></a> is preferred over a custom function that
simply calls this macro.</p>
<p>Memory allocated by this macro must be freed with
<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> (usually called via the object’s
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_free" title="PyTypeObject.tp_free"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_free</span></code></a> slot).</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<ul class="simple">
<li><p><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></li>
<li><p><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></p></li>
<li><p><a class="reference internal" href="type.html#c.PyType_GenericAlloc" title="PyType_GenericAlloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GenericAlloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_alloc" title="PyTypeObject.tp_alloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_alloc</span></code></a></p></li>
</ul>
</div>
</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>
<p>Memory allocated by this function must be freed with
<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> (usually called via the object’s
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_free" title="PyTypeObject.tp_free"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_free</span></code></a> slot).</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>
<p>Do not call this directly to free an object’s memory; call the type’s
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_free" title="PyTypeObject.tp_free"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_free</span></code></a> slot instead.</p>
<p>Do not use this for memory allocated by <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>,
<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>, or related allocation functions; use
<a class="reference internal" href="memory.html#c.PyObject_Free" title="PyObject_Free"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Free()</span></code></a> instead.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<ul class="simple">
<li><p><a class="reference internal" href="memory.html#c.PyObject_Free" title="PyObject_Free"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_Free()</span></code></a> is the non-GC equivalent of this function.</p></li>
<li><p><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></p></li>
<li><p><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><a class="reference internal" href="type.html#c.PyType_GenericAlloc" title="PyType_GenericAlloc"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GenericAlloc()</span></code></a></p></li>
<li><p><a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_free" title="PyTypeObject.tp_free"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_free</span></code></a></p></li>
</ul>
</div>
</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 <code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code> 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_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="traversal">
<span id="gc-traversal"></span><h2>Traversal<a class="headerlink" href="#traversal" title="Link to this heading">¶</a></h2>
<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 garbage-collected object, used by the garbage
collector to detect reference cycles.
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>
<p>A typical <code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code> function calls the <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>
convenience macro on each of the instance’s members that are Python
objects that the instance owns.
For example, this is a (slightly outdated) traversal function for
the <a class="reference internal" href="../library/threading.html#threading.local" title="threading.local"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.local</span></code></a> class:</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">local_traverse</span><span class="p">(</span><span class="n">PyObject</span><span class="w"> </span><span class="o">*</span><span class="n">op</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">localobject</span><span class="w"> </span><span class="o">*</span><span class="n">self</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="p">(</span><span class="n">localobject</span><span class="w"> </span><span class="o">*</span><span class="p">)</span><span class="w"> </span><span class="n">op</span><span class="p">;</span>
<span class="w"> </span><span class="n">Py_VISIT</span><span class="p">(</span><span class="n">Py_TYPE</span><span class="p">(</span><span class="n">self</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">-></span><span class="n">args</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">-></span><span class="n">kw</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">-></span><span class="n">dict</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>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p><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> requires the <em>visit</em> and <em>arg</em> parameters to
<code class="xref c c-func docutils literal notranslate"><span class="pre">local_traverse()</span></code> to have these specific names; don’t name them just
anything.</p>
</div>
<p>Instances of <a class="reference internal" href="typeobj.html#heap-types"><span class="std std-ref">heap-allocated types</span></a> hold a reference to
their type. Their traversal function must therefore visit the type:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">Py_VISIT</span><span class="p">(</span><span class="n">Py_TYPE</span><span class="p">(</span><span class="n">self</span><span class="p">));</span>
</pre></div>
</div>
<p>Alternately, the type may delegate this responsibility by
calling <code class="docutils literal notranslate"><span class="pre">tp_traverse</span></code> of a heap-allocated superclass (or another
heap-allocated type, if applicable).
If they do not, the type object may not be garbage-collected.</p>
<p>If the <a class="reference internal" href="typeobj.html#c.Py_TPFLAGS_MANAGED_DICT" title="Py_TPFLAGS_MANAGED_DICT"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_TPFLAGS_MANAGED_DICT</span></code></a> bit is set in 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, the traverse function must call
<a class="reference internal" href="object.html#c.PyObject_VisitManagedDict" title="PyObject_VisitManagedDict"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_VisitManagedDict()</span></code></a> like this:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="kt">int</span><span class="w"> </span><span class="n">err</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">PyObject_VisitManagedDict</span><span class="p">((</span><span class="n">PyObject</span><span class="o">*</span><span class="p">)</span><span class="n">self</span><span class="p">,</span><span class="w"> </span><span class="n">visit</span><span class="p">,</span><span class="w"> </span><span class="n">arg</span><span class="p">);</span>
<span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">err</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">err</span><span class="p">;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>Only the members that the instance <em>owns</em> (by having
<a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong references</span></a> to them) must be
visited. For instance, if an object supports weak references via the
<a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_weaklist" title="PyTypeObject.tp_weaklist"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_weaklist</span></code></a> slot, the pointer supporting
the linked list (what <em>tp_weaklist</em> points to) must <strong>not</strong> be
visited as the instance does not directly own the weak references to itself.</p>
<p>The traversal function has a limitation:</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>The traversal function must not have any side effects. Implementations
may not modify the reference counts of any Python objects nor create or
destroy any Python objects, directly or indirectly.</p>
</div>
<p>This means that <em>most</em> Python C API functions may not be used, since
they can raise a new exception, return a new reference to a result object,
have internal logic that uses side effects.
Also, unless documented otherwise, functions that happen to not have side
effects may start having them in future versions, without warning.</p>
<p>For a list of safe functions, see a
<a class="reference internal" href="#duringgc-functions"><span class="std std-ref">separate section</span></a> below.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>The <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> call may be skipped for those members that provably
cannot participate in reference cycles.
In the <code class="docutils literal notranslate"><span class="pre">local_traverse</span></code> example above, there is also a <code class="docutils literal notranslate"><span class="pre">self->key</span></code>
member, but it can only be <code class="docutils literal notranslate"><span class="pre">NULL</span></code> or a Python string and therefore
cannot be part of a reference cycle.</p>
<p>On the other hand, even if you know a member can never be part of a cycle,
as a debugging aid you may want to visit it anyway just so the <a class="reference internal" href="../library/gc.html#module-gc" title="gc: Interface to the cycle-detecting garbage collector."><code class="xref py py-mod docutils literal notranslate"><span class="pre">gc</span></code></a>
module’s <a class="reference internal" href="../library/gc.html#gc.get_referents" title="gc.get_referents"><code class="xref py py-func docutils literal notranslate"><span class="pre">get_referents()</span></code></a> function will include it.</p>
</div>
<div class="admonition note">
<p class="admonition-title">Note</p>
<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> function can be called from any
thread.</p>
<div class="impl-detail compound">
<p><strong>CPython implementation detail:</strong> Garbage collection is a “stop-the-world” operation:
even in <a class="reference internal" href="../glossary.html#term-free-threading"><span class="xref std std-term">free threading</span></a> builds, only one thread state is
<a class="reference internal" href="../glossary.html#term-attached-thread-state"><span class="xref std std-term">attached</span></a> when <code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code>
handlers run.</p>
</div>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.9: </span>Heap-allocated types are expected to visit <code class="docutils literal notranslate"><span class="pre">Py_TYPE(self)</span></code> in
<code class="docutils literal notranslate"><span class="pre">tp_traverse</span></code>. In earlier versions of Python, due to
<a class="reference external" href="https://bugs.python.org/issue40217">bug 40217</a>, doing this
may lead to crashes in subclasses.</p>
</div>
</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 <code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code>
implementation must name its arguments exactly <em>visit</em> and <em>arg</em>:</p>
<dl class="c macro">
<dt class="sig sig-object c" id="c.Py_VISIT">
<span class="sig-name descname"><span class="n"><span class="pre">Py_VISIT</span></span></span><span class="sig-paren">(</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 the <span class="c-expr sig sig-inline c"><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n">PyObject</span></a><span class="p">*</span></span> <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.</p>
<p>This corresponds roughly to:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="cp">#define Py_VISIT(o) \</span>
<span class="cp"> if (op) { \</span>
<span class="cp"> int visit_result = visit(o, arg); \</span>
<span class="cp"> if (visit_result != 0) { \</span>
<span class="cp"> return visit_result; \</span>
<span class="cp"> } \</span>
<span class="cp"> }</span>
</pre></div>
</div>
</dd></dl>
<section id="traversal-safe-functions">
<h3>Traversal-safe functions<a class="headerlink" href="#traversal-safe-functions" title="Link to this heading">¶</a></h3>
<p>The following functions and macros are safe to use in 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:</p>
<ul class="simple">
<li><p>the <em>visit</em> function passed to <code class="docutils literal notranslate"><span class="pre">tp_traverse</span></code></p></li>
<li><p><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></p></li>
<li><p><a class="reference internal" href="structures.html#c.Py_SIZE" title="Py_SIZE"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_SIZE()</span></code></a></p></li>
<li><p><a class="reference internal" href="structures.html#c.Py_TYPE" title="Py_TYPE"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_TYPE()</span></code></a>: if called from a <code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code> handler,
<code class="xref c c-func docutils literal notranslate"><span class="pre">Py_TYPE()</span></code>’s result will be valid for the duration of the handler call</p></li>
<li><p><a class="reference internal" href="object.html#c.PyObject_VisitManagedDict" title="PyObject_VisitManagedDict"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_VisitManagedDict()</span></code></a></p></li>
<li><p><a class="reference internal" href="object.html#c.PyObject_TypeCheck" title="PyObject_TypeCheck"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_TypeCheck()</span></code></a>, <a class="reference internal" href="type.html#c.PyType_IsSubtype" title="PyType_IsSubtype"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_IsSubtype()</span></code></a>,
<a class="reference internal" href="type.html#c.PyType_HasFeature" title="PyType_HasFeature"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_HasFeature()</span></code></a></p></li>
<li><p><code class="samp docutils literal notranslate"><span class="pre">Py</span><em><span class="pre"><type></span></em><span class="pre">_Check</span></code> and <code class="samp docutils literal notranslate"><span class="pre">Py</span><em><span class="pre"><type></span></em><span class="pre">_CheckExact</span></code> – for example,
<a class="reference internal" href="tuple.html#c.PyTuple_Check" title="PyTuple_Check"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyTuple_Check()</span></code></a></p></li>
<li><p><a class="reference internal" href="#duringgc-functions"><span class="std std-ref">“DuringGC” functions</span></a></p></li>
</ul>
</section>
<section id="duringgc-functions">
<span id="id1"></span><h3>“DuringGC” functions<a class="headerlink" href="#duringgc-functions" title="Link to this heading">¶</a></h3>
<p>The following functions should <em>only</em> be used in 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; calling them in other
contexts may have unintended consequences.</p>
<p>These functions act like their counterparts without the <code class="docutils literal notranslate"><span class="pre">_DuringGC</span></code> suffix,
but they are guaranteed to not have side effects, they do not set an exception
on failure, and they return/set <a class="reference internal" href="../glossary.html#term-borrowed-reference"><span class="xref std std-term">borrowed references</span></a>
as detailed in the individual documentation.</p>
<p>Note that these functions may fail (return <code class="docutils literal notranslate"><span class="pre">NULL</span></code> or <code class="docutils literal notranslate"><span class="pre">-1</span></code>),
but as they do not set an exception, no error information is available.
In some cases, failure is not distinguishable from a successful <code class="docutils literal notranslate"><span class="pre">NULL</span></code> result.</p>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_GetTypeData_DuringGC">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_GetTypeData_DuringGC</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>, <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">cls</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_GetTypeData_DuringGC" title="Link to this definition">¶</a><br /></dt>
<dt class="sig sig-object c" id="c.PyObject_GetItemData_DuringGC">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_GetItemData_DuringGC</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.PyObject_GetItemData_DuringGC" title="Link to this definition">¶</a><br /></dt>
<dt class="sig sig-object c" id="c.PyType_GetModuleState_DuringGC">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyType_GetModuleState_DuringGC</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="sig-paren">)</span><a class="headerlink" href="#c.PyType_GetModuleState_DuringGC" title="Link to this definition">¶</a><br /></dt>
<dt class="sig sig-object c" id="c.PyModule_GetState_DuringGC">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyModule_GetState_DuringGC</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">module</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyModule_GetState_DuringGC" title="Link to this definition">¶</a><br /></dt>
<dt class="sig sig-object c" id="c.PyModule_GetToken_DuringGC">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyModule_GetToken_DuringGC</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">module</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="p"><span class="pre">*</span></span><span class="n"><span class="pre">result</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyModule_GetToken_DuringGC" 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 3.15.</em><p>See <a class="reference internal" href="#duringgc-functions"><span class="std std-ref">“DuringGC” functions</span></a> for common information.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.15.</span></p>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="object.html#c.PyObject_GetTypeData" title="PyObject_GetTypeData"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_GetTypeData()</span></code></a>,
<a class="reference internal" href="object.html#c.PyObject_GetItemData" title="PyObject_GetItemData"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_GetItemData()</span></code></a>,
<a class="reference internal" href="type.html#c.PyType_GetModuleState" title="PyType_GetModuleState"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GetModuleState()</span></code></a>,
<a class="reference internal" href="module.html#c.PyModule_GetState" title="PyModule_GetState"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyModule_GetState()</span></code></a>,
<a class="reference internal" href="module.html#c.PyModule_GetToken" title="PyModule_GetToken"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyModule_GetToken()</span></code></a>,
<a class="reference internal" href="type.html#c.PyType_GetBaseByToken" title="PyType_GetBaseByToken"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GetBaseByToken()</span></code></a></p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyType_GetBaseByToken_DuringGC">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyType_GetBaseByToken_DuringGC</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="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">tp_token</span></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="p"><span class="pre">*</span></span><span class="n"><span class="pre">result</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyType_GetBaseByToken_DuringGC" 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 3.15.</em><p>See <a class="reference internal" href="#duringgc-functions"><span class="std std-ref">“DuringGC” functions</span></a> for common information.</p>
<p>Sets <em>*result</em> to a <a class="reference internal" href="../glossary.html#term-borrowed-reference"><span class="xref std std-term">borrowed reference</span></a> rather than a strong one.
The reference is valid for the duration
of the <code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code> handler call.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.15.</span></p>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="type.html#c.PyType_GetBaseByToken" title="PyType_GetBaseByToken"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GetBaseByToken()</span></code></a></p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyType_GetModule_DuringGC">
<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">PyType_GetModule_DuringGC</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="sig-paren">)</span><a class="headerlink" href="#c.PyType_GetModule_DuringGC" title="Link to this definition">¶</a><br /></dt>
<dt class="sig sig-object c" id="c.PyType_GetModuleByToken_DuringGC">
<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">PyType_GetModuleByToken_DuringGC</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="k"><span class="pre">const</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">mod_token</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyType_GetModuleByToken_DuringGC" title="Link to this definition">¶</a><br /></dt>
<dd><em class="refcount return_borrowed_ref">Return value: Borrowed reference.</em><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 3.15.</em><p>See <a class="reference internal" href="#duringgc-functions"><span class="std std-ref">“DuringGC” functions</span></a> for common information.</p>
<p>These functions return a <a class="reference internal" href="../glossary.html#term-borrowed-reference"><span class="xref std std-term">borrowed reference</span></a>, which is
valid for the duration of the <code class="xref c c-member docutils literal notranslate"><span class="pre">tp_traverse</span></code> handler call.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.15.</span></p>
</div>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="type.html#c.PyType_GetModule" title="PyType_GetModule"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GetModule()</span></code></a>,
<a class="reference internal" href="type.html#c.PyType_GetModuleByToken" title="PyType_GetModuleByToken"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_GetModuleByToken()</span></code></a></p>
</div>
</dd></dl>
</section>
</section>
<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">1</span></code> to continue iteration, return <code class="docutils literal notranslate"><span class="pre">0</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>
<div class="clearer"></div>
</div>
</div>
</div>
<div class="sphinxsidebar" role="navigation" aria-label="Main">
<div class="sphinxsidebarwrapper">
<div>
<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="#traversal">Traversal</a><ul>
<li><a class="reference internal" href="#traversal-safe-functions">Traversal-safe functions</a></li>
<li><a class="reference internal" href="#duringgc-functions">“DuringGC” functions</a></li>
</ul>
</li>
<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>
</div>
<div>
<h4>Previous topic</h4>
<p class="topless"><a href="typeobj.html"
title="previous chapter">Type Object Structures</a></p>
</div>
<div>
<h4>Next topic</h4>
<p class="topless"><a href="apiabiversion.html"
title="next chapter">API and ABI Versioning</a></p>
</div>
<script>
document.addEventListener('DOMContentLoaded', () => {
const title = document.querySelector('meta[property="og:title"]').content;
const elements = document.querySelectorAll('.improvepage');
const pageurl = window.location.href.split('?')[0];
elements.forEach(element => {
const url = new URL(element.href.split('?')[0].replace("-nojs", ""));
url.searchParams.set('pagetitle', title);
url.searchParams.set('pageurl', pageurl);
url.searchParams.set('pagesource', "c-api/gcsupport.rst");
element.href = url.toString();
});
});
</script>
<div role="note" aria-label="source link">
<h3>This page</h3>
<ul class="this-page-menu">
<li><a href="../bugs.html">Report a bug</a></li>
<li><a class="improvepage" href="../improve-page-nojs.html">Improve this page</a></li>
<li>
<a href="https://github.com/python/cpython/blob/main/Doc/c-api/gcsupport.rst?plain=1"
rel="nofollow">Show source
</a>
</li>
</ul>
</div>
</div>
<div id="sidebarbutton" title="Collapse sidebar">
<span>«</span>
</div>
</div>
<div class="clearer"></div>
</div>
<div class="related" role="navigation" aria-label="Related">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="../genindex.html" title="General Index"
>index</a></li>
<li class="right" >
<a href="../py-modindex.html" title="Python Module Index"
>modules</a> |</li>
<li class="right" >
<a href="apiabiversion.html" title="API and ABI Versioning"
>next</a> |</li>
<li class="right" >
<a href="typeobj.html" title="Type Object Structures"
>previous</a> |</li>
<li><img src="../_static/py.svg" alt="Python logo" style="vertical-align: middle; margin-top: -1px"></li>
<li><a href="https://www.python.org/">Python</a> »</li>
<li class="switchers">
<div class="language_switcher_placeholder"></div>
<div class="version_switcher_placeholder"></div>
</li>
<li>
</li>
<li id="cpython-language-and-version">
<a href="../index.html">3.15.0rc1 Documentation</a> »
</li>
<li class="nav-item nav-item-1"><a href="index.html" >Python/C API reference manual</a> »</li>
<li class="nav-item nav-item-2"><a href="objimpl.html" >Object Implementation Support</a> »</li>
<li class="nav-item nav-item-this"><a href="">Supporting Cyclic Garbage Collection</a></li>
<li class="right">
<div class="inline-search" role="search">
<form class="inline-search" action="../search.html" method="get">
<input placeholder="Quick search" aria-label="Quick search" type="search" name="q" id="search-box">
<input type="submit" value="Go">
</form>
</div>
|
</li>
<li class="right">
<label class="theme-selector-label">
Theme
<select class="theme-selector" oninput="activateTheme(this.value)">
<option value="auto" selected>Auto</option>
<option value="light">Light</option>
<option value="dark">Dark</option>
</select>
</label> |</li>
</ul>
</div>
<div class="footer">
© <a href="../copyright.html">Copyright</a> 2001 Python Software Foundation.
<br>
This page is licensed under the Python Software Foundation License Version 2.
<br>
Examples, recipes, and other code in the documentation are additionally licensed under the Zero Clause BSD License.
<br>
See <a href="/license.html">History and License</a> for more information.<br>
<br>
The Python Software Foundation is a non-profit corporation.
<a href="https://www.python.org/psf/donations/">Please donate.</a>
<br>
<br>
Last updated on Aug 04, 2026 (11:13 UTC).
<a href="/bugs.html">Found a bug</a>?
<br>
Created using <a href="https://www.sphinx-doc.org/">Sphinx</a> 8.2.3.
</div>
</body>
</html>