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<H2><A NAME="SECTION009580000000000000000"></A><A NAME="weakref-objects"></A>
<BR>
7.5.8 Weak Reference Objects
</H2>
<P>
Python supports <em>weak references</em> as first-class objects. There
are two specific object types which directly implement weak
references. The first is a simple reference object, and the second
acts as a proxy for the original object as much as it can.
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>int <b><tt id='l2h-717' xml:id='l2h-717' class="cfunction">PyWeakref_Check</tt></b>(</nobr></td><td>ob)</td></tr></table></dt>
<dd>
Return true if <var>ob</var> is either a reference or proxy object.
<span class="versionnote">New in version 2.2.</span>
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>int <b><tt id='l2h-718' xml:id='l2h-718' class="cfunction">PyWeakref_CheckRef</tt></b>(</nobr></td><td>ob)</td></tr></table></dt>
<dd>
Return true if <var>ob</var> is a reference object.
<span class="versionnote">New in version 2.2.</span>
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>int <b><tt id='l2h-719' xml:id='l2h-719' class="cfunction">PyWeakref_CheckProxy</tt></b>(</nobr></td><td>ob)</td></tr></table></dt>
<dd>
Return true if <var>ob</var> is a proxy object.
<span class="versionnote">New in version 2.2.</span>
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject* <b><tt id='l2h-720' xml:id='l2h-720' class="cfunction">PyWeakref_NewRef</tt></b>(</nobr></td><td>PyObject *<var>ob</var>,
PyObject *<var>callback</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
<span class="label">Return value:</span>
<span class="value">New reference.</span>
</div>
Return a weak reference object for the object <var>ob</var>. This will
always return a new reference, but is not guaranteed to create a new
object; an existing reference object may be returned. The second
parameter, <var>callback</var>, can be a callable object that receives
notification when <var>ob</var> is garbage collected; it should accept a
single parameter, which will be the weak reference object itself.
<var>callback</var> may also be <code>None</code> or <tt class="constant">NULL</tt>. If <var>ob</var>
is not a weakly-referencable object, or if <var>callback</var> is not
callable, <code>None</code>, or <tt class="constant">NULL</tt>, this will return <tt class="constant">NULL</tt> and
raise <tt class="exception">TypeError</tt>.
<span class="versionnote">New in version 2.2.</span>
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject* <b><tt id='l2h-721' xml:id='l2h-721' class="cfunction">PyWeakref_NewProxy</tt></b>(</nobr></td><td>PyObject *<var>ob</var>,
PyObject *<var>callback</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
<span class="label">Return value:</span>
<span class="value">New reference.</span>
</div>
Return a weak reference proxy object for the object <var>ob</var>. This
will always return a new reference, but is not guaranteed to create
a new object; an existing proxy object may be returned. The second
parameter, <var>callback</var>, can be a callable object that receives
notification when <var>ob</var> is garbage collected; it should accept a
single parameter, which will be the weak reference object itself.
<var>callback</var> may also be <code>None</code> or <tt class="constant">NULL</tt>. If <var>ob</var> is not
a weakly-referencable object, or if <var>callback</var> is not callable,
<code>None</code>, or <tt class="constant">NULL</tt>, this will return <tt class="constant">NULL</tt> and raise
<tt class="exception">TypeError</tt>.
<span class="versionnote">New in version 2.2.</span>
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject* <b><tt id='l2h-722' xml:id='l2h-722' class="cfunction">PyWeakref_GetObject</tt></b>(</nobr></td><td>PyObject *<var>ref</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
<span class="label">Return value:</span>
<span class="value">Borrowed reference.</span>
</div>
Return the referenced object from a weak reference, <var>ref</var>. If
the referent is no longer live, returns <code>None</code>.
<span class="versionnote">New in version 2.2.</span>
</dd></dl>
<P>
<dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"><td><nobr>PyObject* <b><tt id='l2h-723' xml:id='l2h-723' class="cfunction">PyWeakref_GET_OBJECT</tt></b>(</nobr></td><td>PyObject *<var>ref</var>)</td></tr></table></dt>
<dd>
<div class="refcount-info">
<span class="label">Return value:</span>
<span class="value">Borrowed reference.</span>
</div>
Similar to <tt class="cfunction">PyWeakref_GetObject()</tt>, but implemented as a
macro that does no error checking.
<span class="versionnote">New in version 2.2.</span>
</dd></dl>
<P>
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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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