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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&nbsp;<b><tt id='l2h-718' xml:id='l2h-718' 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&nbsp;<b><tt id='l2h-719' xml:id='l2h-719' 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&nbsp;<b><tt id='l2h-720' xml:id='l2h-720' 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*&nbsp;<b><tt id='l2h-721' xml:id='l2h-721' 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*&nbsp;<b><tt id='l2h-722' xml:id='l2h-722' 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*&nbsp;<b><tt id='l2h-723' xml:id='l2h-723' 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*&nbsp;<b><tt id='l2h-724' xml:id='l2h-724' 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 2.4.2, documentation updated on 28 September 2005.</span>
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