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  <div class="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="function">
<dt id="Py_INCREF">
void <tt class="descname">Py_INCREF</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em>&nbsp;*o</em><big>)</big><a class="headerlink" href="#Py_INCREF" title="Permalink to this definition">¶</a></dt>
<dd><p>Increment the reference count for object <em>o</em>.  The object must not be <em>NULL</em>; if
you aren&#8217;t sure that it isn&#8217;t <em>NULL</em>, use <a class="reference internal" href="#Py_XINCREF" title="Py_XINCREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_XINCREF()</span></tt></a>.</p>
</dd></dl>

<dl class="function">
<dt id="Py_XINCREF">
void <tt class="descname">Py_XINCREF</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em>&nbsp;*o</em><big>)</big><a class="headerlink" href="#Py_XINCREF" title="Permalink to this definition">¶</a></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="function">
<dt id="Py_DECREF">
void <tt class="descname">Py_DECREF</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em>&nbsp;*o</em><big>)</big><a class="headerlink" href="#Py_DECREF" title="Permalink to this definition">¶</a></dt>
<dd><p>Decrement the reference count for object <em>o</em>.  The object must not be <em>NULL</em>; if
you aren&#8217;t sure that it isn&#8217;t <em>NULL</em>, use <a class="reference internal" href="#Py_XDECREF" title="Py_XDECREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_XDECREF()</span></tt></a>.  If the reference
count reaches zero, the object&#8217;s type&#8217;s deallocation function (which must not be
<em>NULL</em>) is invoked.</p>
<div class="admonition warning">
<p class="first admonition-title">Warning</p>
<p class="last">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__"><tt class="xref py py-meth docutils literal"><span class="pre">__del__()</span></tt></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="#Py_DECREF" title="Py_DECREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_DECREF()</span></tt></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="#Py_DECREF" title="Py_DECREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_DECREF()</span></tt></a> for the temporary variable.</p>
</div>
</dd></dl>

<dl class="function">
<dt id="Py_XDECREF">
void <tt class="descname">Py_XDECREF</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em>&nbsp;*o</em><big>)</big><a class="headerlink" href="#Py_XDECREF" title="Permalink to this definition">¶</a></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="#Py_DECREF" title="Py_DECREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_DECREF()</span></tt></a>, and the same warning applies.</p>
</dd></dl>

<dl class="function">
<dt id="Py_CLEAR">
void <tt class="descname">Py_CLEAR</tt><big>(</big><a class="reference internal" href="structures.html#PyObject" title="PyObject">PyObject</a><em>&nbsp;*o</em><big>)</big><a class="headerlink" href="#Py_CLEAR" title="Permalink to this definition">¶</a></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="#Py_DECREF" title="Py_DECREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_DECREF()</span></tt></a>, except that the argument is also set to <em>NULL</em>.  The warning
for <a class="reference internal" href="#Py_DECREF" title="Py_DECREF"><tt class="xref c c-func docutils literal"><span class="pre">Py_DECREF()</span></tt></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>
</dd></dl>

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


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