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  <section id="reference-counting">
<span id="countingrefs"></span><h1>Reference Counting<a class="headerlink" href="#reference-counting" title="Permalink to this headline">¶</a></h1>
<p>The functions and macros in this section are used for managing reference counts
of Python objects.</p>
<dl class="c function">
<dt class="sig sig-object c" id="c.Py_REFCNT">
<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">Py_REFCNT</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_REFCNT" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Get the reference count of the Python object <em>o</em>.</p>
<p>Note that the returned value may not actually reflect how many
references to the object are actually held.  For example, some
objects are “immortal” and have a very high refcount that does not
reflect the actual number of references.  Consequently, do not rely
on the returned value to be accurate, other than a value of 0 or 1.</p>
<p>Use the <a class="reference internal" href="#c.Py_SET_REFCNT" title="Py_SET_REFCNT"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_SET_REFCNT()</span></code></a> function to set an object reference count.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.11: </span>The parameter type is no longer <span class="c-expr sig sig-inline c"><span class="k">const</span><span class="w"> </span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n">PyObject</span></a><span class="p">*</span></span>.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.10: </span><a class="reference internal" href="#c.Py_REFCNT" title="Py_REFCNT"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_REFCNT()</span></code></a> is changed to the inline static function.</p>
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_SET_REFCNT">
<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_SET_REFCNT</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="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="n"><span class="pre">refcnt</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.Py_SET_REFCNT" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Set the object <em>o</em> reference counter to <em>refcnt</em>.</p>
<p>Note that this function has no effect on
<a class="reference external" href="https://peps.python.org/pep-0683/">immortal</a>
objects.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.9.</span></p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.12: </span>Immortal objects are not modified.</p>
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_INCREF">
<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_INCREF</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_INCREF" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Indicate taking a new <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> to object <em>o</em>,
indicating it is in use and should not be destroyed.</p>
<p>This function is usually used to convert a <a class="reference internal" href="../glossary.html#term-borrowed-reference"><span class="xref std std-term">borrowed reference</span></a> to a
<a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> in-place. The <a class="reference internal" href="#c.Py_NewRef" title="Py_NewRef"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_NewRef()</span></code></a> function can be
used to create a new <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a>.</p>
<p>When done using the object, release it by calling <a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>.</p>
<p>The object must not be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>; if you aren’t sure that it isn’t
<code class="docutils literal notranslate"><span class="pre">NULL</span></code>, use <a class="reference internal" href="#c.Py_XINCREF" title="Py_XINCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XINCREF()</span></code></a>.</p>
<p>Do not expect this function to actually modify <em>o</em> in any way.
For at least <a class="reference external" href="https://peps.python.org/pep-0683/">some objects</a>,
this function has no effect.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.12: </span>Immortal objects are not modified.</p>
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_XINCREF">
<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_XINCREF</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_XINCREF" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Similar to <a class="reference internal" href="#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a>, but the object <em>o</em> can be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>,
in which case this has no effect.</p>
<p>See also <a class="reference internal" href="#c.Py_XNewRef" title="Py_XNewRef"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XNewRef()</span></code></a>.</p>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_NewRef">
<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">Py_NewRef</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_NewRef" title="Permalink 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>Create a new <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> to an object:
call <a class="reference internal" href="#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a> on <em>o</em> and return the object <em>o</em>.</p>
<p>When the <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> is no longer needed, <a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>
should be called on it to release the reference.</p>
<p>The object <em>o</em> must not be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>; use <a class="reference internal" href="#c.Py_XNewRef" title="Py_XNewRef"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XNewRef()</span></code></a> if <em>o</em> can be
<code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<p>For example:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">Py_INCREF</span><span class="p">(</span><span class="n">obj</span><span class="p">);</span>
<span class="n">self</span><span class="o">-&gt;</span><span class="n">attr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">obj</span><span class="p">;</span>
</pre></div>
</div>
<p>can be written as:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">self</span><span class="o">-&gt;</span><span class="n">attr</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">Py_NewRef</span><span class="p">(</span><span class="n">obj</span><span class="p">);</span>
</pre></div>
</div>
<p>See also <a class="reference internal" href="#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.10.</span></p>
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_XNewRef">
<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">Py_XNewRef</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_XNewRef" title="Permalink 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>Similar to <a class="reference internal" href="#c.Py_NewRef" title="Py_NewRef"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_NewRef()</span></code></a>, but the object <em>o</em> can be NULL.</p>
<p>If the object <em>o</em> is <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, the function just returns <code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.10.</span></p>
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_DECREF">
<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_DECREF</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_DECREF" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Release a <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> to object <em>o</em>, indicating the
reference is no longer used.</p>
<p>Once the last <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> is released
(i.e. the object’s reference count reaches 0),
the object’s type’s deallocation
function (which must not be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>) is invoked.</p>
<p>This function is usually used to delete a <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> before
exiting its scope.</p>
<p>The object must not be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>; if you aren’t sure that it isn’t <code class="docutils literal notranslate"><span class="pre">NULL</span></code>,
use <a class="reference internal" href="#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XDECREF()</span></code></a>.</p>
<p>Do not expect this function to actually modify <em>o</em> in any way.
For at least <a class="reference external" href="https://peps.python.org/pep-0683/">some objects</a>,
this function has no effect.</p>
<div class="admonition warning">
<p class="admonition-title">Warning</p>
<p>The deallocation function can cause arbitrary Python code to be invoked (e.g.
when a class instance with a <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a> method is deallocated).  While
exceptions in such code are not propagated, the executed code has free access to
all Python global variables.  This means that any object that is reachable from
a global variable should be in a consistent state before <a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> is
invoked.  For example, code to delete an object from a list should copy a
reference to the deleted object in a temporary variable, update the list data
structure, and then call <a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> for the temporary variable.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.12: </span>Immortal objects are not modified.</p>
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_XDECREF">
<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_XDECREF</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_XDECREF" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Similar to <a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>, but the object <em>o</em> can be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>,
in which case this has no effect.
The same warning from <a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> applies here as well.</p>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_CLEAR">
<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_CLEAR</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_CLEAR" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Release a <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> for object <em>o</em>.
The object may be <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, in
which case the macro has no effect; otherwise the effect is the same as for
<a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>, except that the argument is also set to <code class="docutils literal notranslate"><span class="pre">NULL</span></code>.  The warning
for <a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a> does not apply with respect to the object passed because
the macro carefully uses a temporary variable and sets the argument to <code class="docutils literal notranslate"><span class="pre">NULL</span></code>
before releasing the reference.</p>
<p>It is a good idea to use this macro whenever releasing a reference
to an object that might be traversed during garbage collection.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.12: </span>The macro argument is now only evaluated once. If the argument has side
effects, these are no longer duplicated.</p>
</div>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_IncRef">
<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_IncRef</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_IncRef" title="Permalink 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>Indicate taking a new <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> to object <em>o</em>.
A function version of <a class="reference internal" href="#c.Py_XINCREF" title="Py_XINCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XINCREF()</span></code></a>.
It can be used for runtime dynamic embedding of Python.</p>
</dd></dl>

<dl class="c function">
<dt class="sig sig-object c" id="c.Py_DecRef">
<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_DecRef</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_DecRef" title="Permalink 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>Release a <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> to object <em>o</em>.
A function version of <a class="reference internal" href="#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XDECREF()</span></code></a>.
It can be used for runtime dynamic embedding of Python.</p>
</dd></dl>

<dl class="c macro">
<dt class="sig sig-object c" id="c.Py_SETREF">
<span class="sig-name descname"><span class="n"><span class="pre">Py_SETREF</span></span></span><span class="sig-paren">(</span><span class="n"><span class="pre">dst</span></span>, <span class="n"><span class="pre">src</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.Py_SETREF" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Macro safely releasing a <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> to object <em>dst</em>
and setting <em>dst</em> to <em>src</em>.</p>
<p>As in case of <a class="reference internal" href="#c.Py_CLEAR" title="Py_CLEAR"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_CLEAR()</span></code></a>, “the obvious” code can be deadly:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">Py_DECREF</span><span class="p">(</span><span class="n">dst</span><span class="p">);</span>
<span class="n">dst</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">src</span><span class="p">;</span>
</pre></div>
</div>
<p>The safe way is:</p>
<div class="highlight-c notranslate"><div class="highlight"><pre><span></span><span class="n">Py_SETREF</span><span class="p">(</span><span class="n">dst</span><span class="p">,</span><span class="w"> </span><span class="n">src</span><span class="p">);</span>
</pre></div>
</div>
<p>That arranges to set <em>dst</em> to <em>src</em> _before_ releasing the reference
to the old value of <em>dst</em>, so that any code triggered as a side-effect
of <em>dst</em> getting torn down no longer believes <em>dst</em> points
to a valid object.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6.</span></p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.12: </span>The macro arguments are now only evaluated once. If an argument has side
effects, these are no longer duplicated.</p>
</div>
</dd></dl>

<dl class="c macro">
<dt class="sig sig-object c" id="c.Py_XSETREF">
<span class="sig-name descname"><span class="n"><span class="pre">Py_XSETREF</span></span></span><span class="sig-paren">(</span><span class="n"><span class="pre">dst</span></span>, <span class="n"><span class="pre">src</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.Py_XSETREF" title="Permalink to this definition">¶</a><br /></dt>
<dd><p>Variant of <a class="reference internal" href="#c.Py_SETREF" title="Py_SETREF"><code class="xref c c-macro docutils literal notranslate"><span class="pre">Py_SETREF</span></code></a> macro that uses <a class="reference internal" href="#c.Py_XDECREF" title="Py_XDECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_XDECREF()</span></code></a> instead
of <a class="reference internal" href="#c.Py_DECREF" title="Py_DECREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_DECREF()</span></code></a>.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6.</span></p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.12: </span>The macro arguments are now only evaluated once. If an argument has side
effects, these are no longer duplicated.</p>
</div>
</dd></dl>

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