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<section id="weak-reference-objects">
<span id="weakrefobjects"></span><h1>Weak Reference Objects<a class="headerlink" href="#weak-reference-objects" title="Permalink to this headline"></a></h1>
<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 class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_Check">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyWeakref_Check</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">ob</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyWeakref_Check" title="Permalink to this definition"></a><br /></dt>
<dd><p>Return true if <em>ob</em> is either a reference or proxy object. This function
always succeeds.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_CheckRef">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyWeakref_CheckRef</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">ob</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyWeakref_CheckRef" title="Permalink to this definition"></a><br /></dt>
<dd><p>Return true if <em>ob</em> is a reference object. This function always succeeds.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_CheckProxy">
<span class="kt"><span class="pre">int</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyWeakref_CheckProxy</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">ob</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyWeakref_CheckProxy" title="Permalink to this definition"></a><br /></dt>
<dd><p>Return true if <em>ob</em> is a proxy object. This function always succeeds.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_NewRef">
<a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyWeakref_NewRef</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">ob</span></span>, <a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">callback</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyWeakref_NewRef" title="Permalink to this definition"></a><br /></dt>
<dd><em class="refcount">Return value: New reference.</em><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>Return a weak reference object for the object <em>ob</em>. 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, <em>callback</em>, can be a
callable object that receives notification when <em>ob</em> is garbage collected; it
should accept a single parameter, which will be the weak reference object
itself. <em>callback</em> may also be <code class="docutils literal notranslate"><span class="pre">None</span></code> or <code class="docutils literal notranslate"><span class="pre">NULL</span></code>. If <em>ob</em> is not a
weakly referencable object, or if <em>callback</em> is not callable, <code class="docutils literal notranslate"><span class="pre">None</span></code>, or
<code class="docutils literal notranslate"><span class="pre">NULL</span></code>, this will return <code class="docutils literal notranslate"><span class="pre">NULL</span></code> and raise <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_NewProxy">
<a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyWeakref_NewProxy</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">ob</span></span>, <a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">callback</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyWeakref_NewProxy" title="Permalink to this definition"></a><br /></dt>
<dd><em class="refcount">Return value: New reference.</em><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>Return a weak reference proxy object for the object <em>ob</em>. 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, <em>callback</em>, can
be a callable object that receives notification when <em>ob</em> is garbage
collected; it should accept a single parameter, which will be the weak
reference object itself. <em>callback</em> may also be <code class="docutils literal notranslate"><span class="pre">None</span></code> or <code class="docutils literal notranslate"><span class="pre">NULL</span></code>. If <em>ob</em>
is not a weakly referencable object, or if <em>callback</em> is not callable,
<code class="docutils literal notranslate"><span class="pre">None</span></code>, or <code class="docutils literal notranslate"><span class="pre">NULL</span></code>, this will return <code class="docutils literal notranslate"><span class="pre">NULL</span></code> and raise <a class="reference internal" href="../library/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_GetObject">
<a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyWeakref_GetObject</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">ref</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyWeakref_GetObject" title="Permalink to this definition"></a><br /></dt>
<dd><em class="refcount">Return value: Borrowed reference.</em><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>Return the referenced object from a weak reference, <em>ref</em>. If the referent is
no longer live, returns <code class="docutils literal notranslate"><span class="pre">Py_None</span></code>.</p>
<div class="admonition note">
<p class="admonition-title">Note</p>
<p>This function returns a <a class="reference internal" href="../glossary.html#term-borrowed-reference"><span class="xref std std-term">borrowed reference</span></a> to the referenced object.
This means that you should always call <a class="reference internal" href="refcounting.html#c.Py_INCREF" title="Py_INCREF"><code class="xref c c-func docutils literal notranslate"><span class="pre">Py_INCREF()</span></code></a> on the object
except when it cannot be destroyed before the last usage of the borrowed
reference.</p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_GET_OBJECT">
<a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="sig-name descname"><span class="n"><span class="pre">PyWeakref_GET_OBJECT</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">ref</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyWeakref_GET_OBJECT" title="Permalink to this definition"></a><br /></dt>
<dd><em class="refcount">Return value: Borrowed reference.</em><p>Similar to <a class="reference internal" href="#c.PyWeakref_GetObject" title="PyWeakref_GetObject"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyWeakref_GetObject()</span></code></a>, but does no error checking.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyObject_ClearWeakRefs">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyObject_ClearWeakRefs</span></span></span><span class="sig-paren">(</span><a class="reference internal" href="structures.html#c.PyObject" title="PyObject"><span class="n"><span class="pre">PyObject</span></span></a><span class="w"> </span><span class="p"><span class="pre">*</span></span><span class="n"><span class="pre">object</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyObject_ClearWeakRefs" title="Permalink to this definition"></a><br /></dt>
<dd><em class="stableabi"> Part of the <a class="reference internal" href="stable.html#stable"><span class="std std-ref">Stable ABI</span></a>.</em><p>This function is called by the <a class="reference internal" href="typeobj.html#c.PyTypeObject.tp_dealloc" title="PyTypeObject.tp_dealloc"><code class="xref c c-member docutils literal notranslate"><span class="pre">tp_dealloc</span></code></a> handler
to clear weak references.</p>
<p>This iterates through the weak references for <em>object</em> and calls callbacks
for those references which have one. It returns when all callbacks have
been attempted.</p>
</dd></dl>
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