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<title>7.5.8 Weak Reference Objects
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<H2><A NAME=
"SECTION009580000000000000000"></A><A NAME=
"weakref-objects"></A>
7.5.8 Weak Reference Objects
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.
<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_Check
</tt></b>(
</nobr></td><td>ob)
</td></tr></table></dt>
Return true if
<var>ob
</var> is either a reference or proxy object.
<span class=
"versionnote">New in version
2.2.
</span>
<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_CheckRef
</tt></b>(
</nobr></td><td>ob)
</td></tr></table></dt>
Return true if
<var>ob
</var> is a reference object.
<span class=
"versionnote">New in version
2.2.
</span>
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline"><td><nobr>int
<b><tt id='l2h-
720' xml:id='l2h-
720'
class=
"cfunction">PyWeakref_CheckProxy
</tt></b>(
</nobr></td><td>ob)
</td></tr></table></dt>
Return true if
<var>ob
</var> is a proxy object.
<span class=
"versionnote">New in version
2.2.
</span>
<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_NewRef
</tt></b>(
</nobr></td><td>PyObject *
<var>ob
</var>,
PyObject *
<var>callback
</var>)
</td></tr></table></dt>
<div class=
"refcount-info">
<span class=
"label">Return value:
</span>
<span class=
"value">New reference.
</span>
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>
<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_NewProxy
</tt></b>(
</nobr></td><td>PyObject *
<var>ob
</var>,
PyObject *
<var>callback
</var>)
</td></tr></table></dt>
<div class=
"refcount-info">
<span class=
"label">Return value:
</span>
<span class=
"value">New reference.
</span>
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>
<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_GetObject
</tt></b>(
</nobr></td><td>PyObject *
<var>ref
</var>)
</td></tr></table></dt>
<div class=
"refcount-info">
<span class=
"label">Return value:
</span>
<span class=
"value">Borrowed reference.
</span>
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>
<dl><dt><table cellpadding=
"0" cellspacing=
"0"><tr valign=
"baseline"><td><nobr>PyObject*
<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>
<div class=
"refcount-info">
<span class=
"label">Return value:
</span>
<span class=
"value">Borrowed reference.
</span>
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>
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<span class=
"release-info">Release
2.4.2, documentation updated on
28 September
2005.
</span>
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