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<H1><A NAME="SECTION005000000000000000000"></A><A NAME="countingRefs"></A>
<BR>
3. Reference Counting
</H1>
<P>
The macros in this section are used for managing reference counts
of Python objects.
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-84' xml:id='l2h-84' class="cfunction">Py_INCREF</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt>
<dd>
Increment the reference count for object <var>o</var>. The object must
not be <tt class="constant">NULL</tt>; if you aren't sure that it isn't <tt class="constant">NULL</tt>, use
<tt class="cfunction">Py_XINCREF()</tt>.
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-85' xml:id='l2h-85' class="cfunction">Py_XINCREF</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt>
<dd>
Increment the reference count for object <var>o</var>. The object may be
<tt class="constant">NULL</tt>, in which case the macro has no effect.
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-86' xml:id='l2h-86' class="cfunction">Py_DECREF</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt>
<dd>
Decrement the reference count for object <var>o</var>. The object must
not be <tt class="constant">NULL</tt>; if you aren't sure that it isn't <tt class="constant">NULL</tt>, use
<tt class="cfunction">Py_XDECREF()</tt>. If the reference count reaches zero, the
object's type's deallocation function (which must not be <tt class="constant">NULL</tt>) is
invoked.
<P>
<span class="warning"><b class="label">Warning:</b>
The deallocation function can cause arbitrary Python code
to be invoked (e.g. when a class instance with a <tt class="method">__del__()</tt>
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
<tt class="cfunction">Py_DECREF()</tt> 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
<tt class="cfunction">Py_DECREF()</tt> for the temporary variable.</span>
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-87' xml:id='l2h-87' class="cfunction">Py_XDECREF</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt>
<dd>
Decrement the reference count for object <var>o</var>. The object may be
<tt class="constant">NULL</tt>, in which case the macro has no effect; otherwise the effect
is the same as for <tt class="cfunction">Py_DECREF()</tt>, and the same warning
applies.
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>void <b><tt id='l2h-88' xml:id='l2h-88' class="cfunction">Py_CLEAR</tt></b>(</nobr></td><td>PyObject *<var>o</var>)</td></tr></table></dt>
<dd>
Decrement the reference count for object <var>o</var>. The object may be
<tt class="constant">NULL</tt>, in which case the macro has no effect; otherwise the effect
is the same as for <tt class="cfunction">Py_DECREF()</tt>, except that the argument
is also set to <tt class="constant">NULL</tt>. The warning for <tt class="cfunction">Py_DECREF()</tt> does
not apply with respect to the object passed because the macro
carefully uses a temporary variable and sets the argument to <tt class="constant">NULL</tt>
before decrementing its reference count.
<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>
<span class="versionnote">New in version 2.4.</span>
</dd></dl>
<P>
The following functions are for runtime dynamic embedding of Python:
<tt class="cfunction">Py_IncRef(PyObject *o)</tt>, <tt class="cfunction">Py_DecRef(PyObject *o)</tt>.
They are simply exported function versions of <tt class="cfunction">Py_XINCREF()</tt> and
<tt class="cfunction">Py_XDECREF()</tt>, respectively.
<P>
The following functions or macros are only for use within the
interpreter core: <tt class="cfunction">_Py_Dealloc()</tt>,
<tt class="cfunction">_Py_ForgetReference()</tt>, <tt class="cfunction">_Py_NewReference()</tt>, as
well as the global variable <tt class="cdata">_Py_RefTotal</tt>.
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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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