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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 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_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>Increment the reference count for object <em>o</em>.  The object must not be <em>NULL</em>; if
you aren’t sure that it isn’t <em>NULL</em>, 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>
</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>Increment the reference count for object <em>o</em>.  The object may be <em>NULL</em>, in
which case the macro has no effect.</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><p>Decrement the reference count for object <em>o</em>.  The object must not be <em>NULL</em>; if
you aren’t sure that it isn’t <em>NULL</em>, 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>.  If the reference
count reaches zero, the object’s type’s deallocation function (which must not be
<em>NULL</em>) is invoked.</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 <code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code> 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>
</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>Decrement the reference count for object <em>o</em>.  The object may be <em>NULL</em>, 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>, and the same warning applies.</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>Decrement the reference count for object <em>o</em>.  The object may be <em>NULL</em>, 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 <em>NULL</em>.  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 <em>NULL</em>
before decrementing its reference count.</p>
<p>It is a good idea to use this macro whenever decrementing the value of a
variable that might be traversed during garbage collection.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 2.4.</span></p>
</div>
</dd></dl>

<p>The following functions are for runtime dynamic embedding of Python:
<code class="docutils literal notranslate"><span class="pre">Py_IncRef(PyObject</span> <span class="pre">*o)</span></code>, <code class="docutils literal notranslate"><span class="pre">Py_DecRef(PyObject</span> <span class="pre">*o)</span></code>. They are
simply exported function versions 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> and
<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>, respectively.</p>
<p>The following functions or macros are only for use within the interpreter core:
<code class="xref c c-func docutils literal notranslate"><span class="pre">_Py_Dealloc()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">_Py_ForgetReference()</span></code>, <code class="xref c c-func docutils literal notranslate"><span class="pre">_Py_NewReference()</span></code>,
as well as the global variable <code class="xref c c-data docutils literal notranslate"><span class="pre">_Py_RefTotal</span></code>.</p>
</section>

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