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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="Link to this heading"></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="Link to this definition"></a><br /></dt>
<dd><p>Return non-zero 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="Link to this definition"></a><br /></dt>
<dd><p>Return non-zero if <em>ob</em> is a reference object or a subclass of the reference
type. This function always succeeds.</p>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_CheckRefExact">
<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_CheckRefExact</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_CheckRefExact" title="Link to this definition"></a><br /></dt>
<dd><p>Return non-zero if <em>ob</em> is a reference object, but not a subclass of the
reference type. 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="Link to this definition"></a><br /></dt>
<dd><p>Return non-zero 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="Link to this definition"></a><br /></dt>
<dd><em class="refcount return_new_ref">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 referenceable object, this will raise <a class="reference internal" href="../builtins/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> and return
<code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="type.html#c.PyType_SUPPORTS_WEAKREFS" title="PyType_SUPPORTS_WEAKREFS"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_SUPPORTS_WEAKREFS()</span></code></a> for checking if <em>ob</em> is weakly
referenceable.</p>
</div>
</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="Link to this definition"></a><br /></dt>
<dd><em class="refcount return_new_ref">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>
weakly referenceable object, this will raise <a class="reference internal" href="../builtins/exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a> and return
<code class="docutils literal notranslate"><span class="pre">NULL</span></code>.</p>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<p><a class="reference internal" href="type.html#c.PyType_SUPPORTS_WEAKREFS" title="PyType_SUPPORTS_WEAKREFS"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyType_SUPPORTS_WEAKREFS()</span></code></a> for checking if <em>ob</em> is weakly
referenceable.</p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_GetRef">
<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_GetRef</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>, <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="p"><span class="pre">*</span></span><span class="n"><span class="pre">pobj</span></span><span class="sig-paren">)</span><a class="headerlink" href="#c.PyWeakref_GetRef" title="Link 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> since version 3.13.</em><p>Get a <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> to the referenced object from a weak
reference, <em>ref</em>, into <em>*pobj</em>.</p>
<ul class="simple">
<li><p>On success, set <em>*pobj</em> to a new <a class="reference internal" href="../glossary.html#term-strong-reference"><span class="xref std std-term">strong reference</span></a> to the
referenced object and return 1.</p></li>
<li><p>If the reference is dead, set <em>*pobj</em> to <code class="docutils literal notranslate"><span class="pre">NULL</span></code> and return 0.</p></li>
<li><p>On error, raise an exception and return -1.</p></li>
</ul>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.13.</span></p>
</div>
</dd></dl>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyWeakref_IsDead">
<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_IsDead</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_IsDead" title="Link to this definition"></a><br /></dt>
<dd><p>Test if the weak reference <em>ref</em> is dead. Returns 1 if the reference is
dead, 0 if it is alive, and -1 with an error set if <em>ref</em> is not a weak
reference object.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.14.</span></p>
</div>
</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="Link 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>
<dl class="c function">
<dt class="sig sig-object c" id="c.PyUnstable_Object_ClearWeakRefsNoCallbacks">
<span class="kt"><span class="pre">void</span></span><span class="w"> </span><span class="sig-name descname"><span class="n"><span class="pre">PyUnstable_Object_ClearWeakRefsNoCallbacks</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.PyUnstable_Object_ClearWeakRefsNoCallbacks" title="Link to this definition"></a><br /></dt>
<dd><div class="unstable-c-api warning admonition">
<em>This is <a class="reference internal" href="stable.html#unstable-c-api"><span class="std std-ref">Unstable API</span></a>. It may change without warning in minor releases.</em></div>
<p>Clears the weakrefs for <em>object</em> without calling the callbacks.</p>
<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
for types with finalizers (i.e., <a class="reference internal" href="../reference/datamodel.html#object.__del__" title="object.__del__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code></a>). The handler for
those objects first calls <a class="reference internal" href="#c.PyObject_ClearWeakRefs" title="PyObject_ClearWeakRefs"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_ClearWeakRefs()</span></code></a> to clear weakrefs
and call their callbacks, then the finalizer, and finally this function to
clear any weakrefs that may have been created by the finalizer.</p>
<p>In most circumstances, it’s more appropriate to use
<a class="reference internal" href="#c.PyObject_ClearWeakRefs" title="PyObject_ClearWeakRefs"><code class="xref c c-func docutils literal notranslate"><span class="pre">PyObject_ClearWeakRefs()</span></code></a> to clear weakrefs instead of this function.</p>
<div class="versionadded">
<p><span class="versionmodified added">Added in version 3.13.</span></p>
</div>
</dd></dl>
</section>
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