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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>8.11. weakref — Weak references — Python 2.7.18 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.18 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="8.12. UserDict — Class wrapper for dictionary objects" href="userdict.html" /> <link rel="prev" title="8.10. Queue — A synchronized queue class" href="queue.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/library/weakref.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"> This document is for an old version of Python that is <a href="https://devguide.python.org/devcycle/#end-of-life-branches">no longer supported</a>. You should install the python3 and python3-doc packages and read the <a href="file:///usr/share/doc/python3-doc/html/library/weakref.html"> Python documentation for the Python3 version packaged in this release</a>. </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="userdict.html" title="8.12. UserDict — Class wrapper for dictionary objects" accesskey="N">next</a> |</li> <li class="right" > <a href="queue.html" title="8.10. Queue — A synchronized queue class" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="datatypes.html" accesskey="U">8. Data Types</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="module-weakref"> <span id="weakref-weak-references"></span><h1>8.11. <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate"><span class="pre">weakref</span></code></a> — Weak references<a class="headerlink" href="#module-weakref" title="Permalink to this headline">¶</a></h1> <div class="versionadded"> <p><span class="versionmodified">New in version 2.1.</span></p> </div> <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/2.7/Lib/weakref.py">Lib/weakref.py</a></p> <hr class="docutils" /> <p>The <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate"><span class="pre">weakref</span></code></a> module allows the Python programmer to create <em class="dfn">weak references</em> to objects.</p> <p>In the following, the term <em class="dfn">referent</em> means the object which is referred to by a weak reference.</p> <p>A weak reference to an object is not enough to keep the object alive: when the only remaining references to a referent are weak references, <a class="reference internal" href="../glossary.html#term-garbage-collection"><span class="xref std std-term">garbage collection</span></a> is free to destroy the referent and reuse its memory for something else. A primary use for weak references is to implement caches or mappings holding large objects, where it’s desired that a large object not be kept alive solely because it appears in a cache or mapping.</p> <p>For example, if you have a number of large binary image objects, you may wish to associate a name with each. If you used a Python dictionary to map names to images, or images to names, the image objects would remain alive just because they appeared as values or keys in the dictionaries. The <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakKeyDictionary</span></code></a> and <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code></a> classes supplied by the <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate"><span class="pre">weakref</span></code></a> module are an alternative, using weak references to construct mappings that don’t keep objects alive solely because they appear in the mapping objects. If, for example, an image object is a value in a <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code></a>, then when the last remaining references to that image object are the weak references held by weak mappings, garbage collection can reclaim the object, and its corresponding entries in weak mappings are simply deleted.</p> <p><a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakKeyDictionary</span></code></a> and <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code></a> use weak references in their implementation, setting up callback functions on the weak references that notify the weak dictionaries when a key or value has been reclaimed by garbage collection. Most programs should find that using one of these weak dictionary types is all they need – it’s not usually necessary to create your own weak references directly. The low-level machinery used by the weak dictionary implementations is exposed by the <a class="reference internal" href="#module-weakref" title="weakref: Support for weak references and weak dictionaries."><code class="xref py py-mod docutils literal notranslate"><span class="pre">weakref</span></code></a> module for the benefit of advanced uses.</p> <p>Not all objects can be weakly referenced; those objects which can include class instances, functions written in Python (but not in C), methods (both bound and unbound), sets, frozensets, file objects, <a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">generator</span></a>s, type objects, <code class="xref py py-class docutils literal notranslate"><span class="pre">DBcursor</span></code> objects from the <a class="reference internal" href="bsddb.html#module-bsddb" title="bsddb: Interface to Berkeley DB database library"><code class="xref py py-mod docutils literal notranslate"><span class="pre">bsddb</span></code></a> module, sockets, arrays, deques, regular expression pattern objects, and code objects.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.4: </span>Added support for files, sockets, arrays, and patterns.</p> </div> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.7: </span>Added support for thread.lock, threading.Lock, and code objects.</p> </div> <p>Several built-in types such as <code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code> and <a class="reference internal" href="stdtypes.html#dict" title="dict"><code class="xref py py-class docutils literal notranslate"><span class="pre">dict</span></code></a> do not directly support weak references but can add support through subclassing:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">Dict</span><span class="p">(</span><span class="nb">dict</span><span class="p">):</span> <span class="k">pass</span> <span class="n">obj</span> <span class="o">=</span> <span class="n">Dict</span><span class="p">(</span><span class="n">red</span><span class="o">=</span><span class="mi">1</span><span class="p">,</span> <span class="n">green</span><span class="o">=</span><span class="mi">2</span><span class="p">,</span> <span class="n">blue</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span> <span class="c1"># this object is weak referenceable</span> </pre></div> </div> <div class="impl-detail compound"> <p><strong>CPython implementation detail:</strong> Other built-in types such as <a class="reference internal" href="functions.html#tuple" title="tuple"><code class="xref py py-class docutils literal notranslate"><span class="pre">tuple</span></code></a> and <a class="reference internal" href="functions.html#long" title="long"><code class="xref py py-class docutils literal notranslate"><span class="pre">long</span></code></a> do not support weak references even when subclassed.</p> </div> <p>Extension types can easily be made to support weak references; see <a class="reference internal" href="../extending/newtypes.html#weakref-support"><span class="std std-ref">Weak Reference Support</span></a>.</p> <dl class="class"> <dt id="weakref.ref"> <em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">ref</code><span class="sig-paren">(</span><em>object</em><span class="optional">[</span>, <em>callback</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.ref" title="Permalink to this definition">¶</a></dt> <dd><p>Return a weak reference to <em>object</em>. The original object can be retrieved by calling the reference object if the referent is still alive; if the referent is no longer alive, calling the reference object will cause <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a> to be returned. If <em>callback</em> is provided and not <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>, and the returned weakref object is still alive, the callback will be called when the object is about to be finalized; the weak reference object will be passed as the only parameter to the callback; the referent will no longer be available.</p> <p>It is allowable for many weak references to be constructed for the same object. Callbacks registered for each weak reference will be called from the most recently registered callback to the oldest registered callback.</p> <p>Exceptions raised by the callback will be noted on the standard error output, but cannot be propagated; they are handled in exactly the same way as exceptions raised from an object’s <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> method.</p> <p>Weak references are <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a> if the <em>object</em> is hashable. They will maintain their hash value even after the <em>object</em> was deleted. If <a class="reference internal" href="functions.html#hash" title="hash"><code class="xref py py-func docutils literal notranslate"><span class="pre">hash()</span></code></a> is called the first time only after the <em>object</em> was deleted, the call will raise <a class="reference internal" href="exceptions.html#exceptions.TypeError" title="exceptions.TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</p> <p>Weak references support tests for equality, but not ordering. If the referents are still alive, two references have the same equality relationship as their referents (regardless of the <em>callback</em>). If either referent has been deleted, the references are equal only if the reference objects are the same object.</p> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 2.4: </span>This is now a subclassable type rather than a factory function; it derives from <a class="reference internal" href="functions.html#object" title="object"><code class="xref py py-class docutils literal notranslate"><span class="pre">object</span></code></a>.</p> </div> </dd></dl> <dl class="function"> <dt id="weakref.proxy"> <code class="descclassname">weakref.</code><code class="descname">proxy</code><span class="sig-paren">(</span><em>object</em><span class="optional">[</span>, <em>callback</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.proxy" title="Permalink to this definition">¶</a></dt> <dd><p>Return a proxy to <em>object</em> which uses a weak reference. This supports use of the proxy in most contexts instead of requiring the explicit dereferencing used with weak reference objects. The returned object will have a type of either <code class="docutils literal notranslate"><span class="pre">ProxyType</span></code> or <code class="docutils literal notranslate"><span class="pre">CallableProxyType</span></code>, depending on whether <em>object</em> is callable. Proxy objects are not <a class="reference internal" href="../glossary.html#term-hashable"><span class="xref std std-term">hashable</span></a> regardless of the referent; this avoids a number of problems related to their fundamentally mutable nature, and prevent their use as dictionary keys. <em>callback</em> is the same as the parameter of the same name to the <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-func docutils literal notranslate"><span class="pre">ref()</span></code></a> function.</p> </dd></dl> <dl class="function"> <dt id="weakref.getweakrefcount"> <code class="descclassname">weakref.</code><code class="descname">getweakrefcount</code><span class="sig-paren">(</span><em>object</em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.getweakrefcount" title="Permalink to this definition">¶</a></dt> <dd><p>Return the number of weak references and proxies which refer to <em>object</em>.</p> </dd></dl> <dl class="function"> <dt id="weakref.getweakrefs"> <code class="descclassname">weakref.</code><code class="descname">getweakrefs</code><span class="sig-paren">(</span><em>object</em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.getweakrefs" title="Permalink to this definition">¶</a></dt> <dd><p>Return a list of all weak reference and proxy objects which refer to <em>object</em>.</p> </dd></dl> <dl class="class"> <dt id="weakref.WeakKeyDictionary"> <em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">WeakKeyDictionary</code><span class="sig-paren">(</span><span class="optional">[</span><em>dict</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakKeyDictionary" title="Permalink to this definition">¶</a></dt> <dd><p>Mapping class that references keys weakly. Entries in the dictionary will be discarded when there is no longer a strong reference to the key. This can be used to associate additional data with an object owned by other parts of an application without adding attributes to those objects. This can be especially useful with objects that override attribute accesses.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">Caution: Because a <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakKeyDictionary</span></code></a> is built on top of a Python dictionary, it must not change size when iterating over it. This can be difficult to ensure for a <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakKeyDictionary</span></code></a> because actions performed by the program during iteration may cause items in the dictionary to vanish “by magic” (as a side effect of garbage collection).</p> </div> </dd></dl> <p><a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakKeyDictionary</span></code></a> objects have the following additional methods. These expose the internal references directly. The references are not guaranteed to be “live” at the time they are used, so the result of calling the references needs to be checked before being used. This can be used to avoid creating references that will cause the garbage collector to keep the keys around longer than needed.</p> <dl class="method"> <dt id="weakref.WeakKeyDictionary.iterkeyrefs"> <code class="descclassname">WeakKeyDictionary.</code><code class="descname">iterkeyrefs</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakKeyDictionary.iterkeyrefs" title="Permalink to this definition">¶</a></dt> <dd><p>Return an iterable of the weak references to the keys.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.5.</span></p> </div> </dd></dl> <dl class="method"> <dt id="weakref.WeakKeyDictionary.keyrefs"> <code class="descclassname">WeakKeyDictionary.</code><code class="descname">keyrefs</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakKeyDictionary.keyrefs" title="Permalink to this definition">¶</a></dt> <dd><p>Return a list of weak references to the keys.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.5.</span></p> </div> </dd></dl> <dl class="class"> <dt id="weakref.WeakValueDictionary"> <em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">WeakValueDictionary</code><span class="sig-paren">(</span><span class="optional">[</span><em>dict</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakValueDictionary" title="Permalink to this definition">¶</a></dt> <dd><p>Mapping class that references values weakly. Entries in the dictionary will be discarded when no strong reference to the value exists any more.</p> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">Caution: Because a <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code></a> is built on top of a Python dictionary, it must not change size when iterating over it. This can be difficult to ensure for a <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code></a> because actions performed by the program during iteration may cause items in the dictionary to vanish “by magic” (as a side effect of garbage collection).</p> </div> </dd></dl> <p><a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code></a> objects have the following additional methods. These methods have the same issues as the <code class="xref py py-meth docutils literal notranslate"><span class="pre">iterkeyrefs()</span></code> and <code class="xref py py-meth docutils literal notranslate"><span class="pre">keyrefs()</span></code> methods of <a class="reference internal" href="#weakref.WeakKeyDictionary" title="weakref.WeakKeyDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakKeyDictionary</span></code></a> objects.</p> <dl class="method"> <dt id="weakref.WeakValueDictionary.itervaluerefs"> <code class="descclassname">WeakValueDictionary.</code><code class="descname">itervaluerefs</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakValueDictionary.itervaluerefs" title="Permalink to this definition">¶</a></dt> <dd><p>Return an iterable of the weak references to the values.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.5.</span></p> </div> </dd></dl> <dl class="method"> <dt id="weakref.WeakValueDictionary.valuerefs"> <code class="descclassname">WeakValueDictionary.</code><code class="descname">valuerefs</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakValueDictionary.valuerefs" title="Permalink to this definition">¶</a></dt> <dd><p>Return a list of weak references to the values.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.5.</span></p> </div> </dd></dl> <dl class="class"> <dt id="weakref.WeakSet"> <em class="property">class </em><code class="descclassname">weakref.</code><code class="descname">WeakSet</code><span class="sig-paren">(</span><span class="optional">[</span><em>elements</em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakSet" title="Permalink to this definition">¶</a></dt> <dd><p>Set class that keeps weak references to its elements. An element will be discarded when no strong reference to it exists any more.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 2.7.</span></p> </div> </dd></dl> <dl class="data"> <dt id="weakref.ReferenceType"> <code class="descclassname">weakref.</code><code class="descname">ReferenceType</code><a class="headerlink" href="#weakref.ReferenceType" title="Permalink to this definition">¶</a></dt> <dd><p>The type object for weak references objects.</p> </dd></dl> <dl class="data"> <dt id="weakref.ProxyType"> <code class="descclassname">weakref.</code><code class="descname">ProxyType</code><a class="headerlink" href="#weakref.ProxyType" title="Permalink to this definition">¶</a></dt> <dd><p>The type object for proxies of objects which are not callable.</p> </dd></dl> <dl class="data"> <dt id="weakref.CallableProxyType"> <code class="descclassname">weakref.</code><code class="descname">CallableProxyType</code><a class="headerlink" href="#weakref.CallableProxyType" title="Permalink to this definition">¶</a></dt> <dd><p>The type object for proxies of callable objects.</p> </dd></dl> <dl class="data"> <dt id="weakref.ProxyTypes"> <code class="descclassname">weakref.</code><code class="descname">ProxyTypes</code><a class="headerlink" href="#weakref.ProxyTypes" title="Permalink to this definition">¶</a></dt> <dd><p>Sequence containing all the type objects for proxies. This can make it simpler to test if an object is a proxy without being dependent on naming both proxy types.</p> </dd></dl> <dl class="exception"> <dt id="weakref.ReferenceError"> <em class="property">exception </em><code class="descclassname">weakref.</code><code class="descname">ReferenceError</code><a class="headerlink" href="#weakref.ReferenceError" title="Permalink to this definition">¶</a></dt> <dd><p>Exception raised when a proxy object is used but the underlying object has been collected. This is the same as the standard <a class="reference internal" href="#weakref.ReferenceError" title="weakref.ReferenceError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ReferenceError</span></code></a> exception.</p> </dd></dl> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-0205"><strong>PEP 205</strong></a> - Weak References</dt> <dd>The proposal and rationale for this feature, including links to earlier implementations and information about similar features in other languages.</dd> </dl> </div> <div class="section" id="weak-reference-objects"> <span id="weakref-objects"></span><h2>8.11.1. Weak Reference Objects<a class="headerlink" href="#weak-reference-objects" title="Permalink to this headline">¶</a></h2> <p>Weak reference objects have no attributes or methods, but do allow the referent to be obtained, if it still exists, by calling it:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="kn">import</span> <span class="nn">weakref</span> <span class="gp">>>> </span><span class="k">class</span> <span class="nc">Object</span><span class="p">:</span> <span class="gp">... </span> <span class="k">pass</span> <span class="gp">...</span> <span class="gp">>>> </span><span class="n">o</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">r</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">ref</span><span class="p">(</span><span class="n">o</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">o2</span> <span class="o">=</span> <span class="n">r</span><span class="p">()</span> <span class="gp">>>> </span><span class="n">o</span> <span class="ow">is</span> <span class="n">o2</span> <span class="go">True</span> </pre></div> </div> <p>If the referent no longer exists, calling the reference object returns <a class="reference internal" href="constants.html#None" title="None"><code class="xref py py-const docutils literal notranslate"><span class="pre">None</span></code></a>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="k">del</span> <span class="n">o</span><span class="p">,</span> <span class="n">o2</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">r</span><span class="p">()</span> <span class="go">None</span> </pre></div> </div> <p>Testing that a weak reference object is still live should be done using the expression <code class="docutils literal notranslate"><span class="pre">ref()</span> <span class="pre">is</span> <span class="pre">not</span> <span class="pre">None</span></code>. Normally, application code that needs to use a reference object should follow this pattern:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># r is a weak reference object</span> <span class="n">o</span> <span class="o">=</span> <span class="n">r</span><span class="p">()</span> <span class="k">if</span> <span class="n">o</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span> <span class="c1"># referent has been garbage collected</span> <span class="nb">print</span> <span class="s2">"Object has been deallocated; can't frobnicate."</span> <span class="k">else</span><span class="p">:</span> <span class="nb">print</span> <span class="s2">"Object is still live!"</span> <span class="n">o</span><span class="o">.</span><span class="n">do_something_useful</span><span class="p">()</span> </pre></div> </div> <p>Using a separate test for “liveness” creates race conditions in threaded applications; another thread can cause a weak reference to become invalidated before the weak reference is called; the idiom shown above is safe in threaded applications as well as single-threaded applications.</p> <p>Specialized versions of <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-class docutils literal notranslate"><span class="pre">ref</span></code></a> objects can be created through subclassing. This is used in the implementation of the <a class="reference internal" href="#weakref.WeakValueDictionary" title="weakref.WeakValueDictionary"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakValueDictionary</span></code></a> to reduce the memory overhead for each entry in the mapping. This may be most useful to associate additional information with a reference, but could also be used to insert additional processing on calls to retrieve the referent.</p> <p>This example shows how a subclass of <a class="reference internal" href="#weakref.ref" title="weakref.ref"><code class="xref py py-class docutils literal notranslate"><span class="pre">ref</span></code></a> can be used to store additional information about an object and affect the value that’s returned when the referent is accessed:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">weakref</span> <span class="k">class</span> <span class="nc">ExtendedRef</span><span class="p">(</span><span class="n">weakref</span><span class="o">.</span><span class="n">ref</span><span class="p">):</span> <span class="k">def</span> <span class="nf">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">ob</span><span class="p">,</span> <span class="n">callback</span><span class="o">=</span><span class="kc">None</span><span class="p">,</span> <span class="o">**</span><span class="n">annotations</span><span class="p">):</span> <span class="nb">super</span><span class="p">(</span><span class="n">ExtendedRef</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__init__</span><span class="p">(</span><span class="n">ob</span><span class="p">,</span> <span class="n">callback</span><span class="p">)</span> <span class="bp">self</span><span class="o">.</span><span class="n">__counter</span> <span class="o">=</span> <span class="mi">0</span> <span class="k">for</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span> <span class="ow">in</span> <span class="n">annotations</span><span class="o">.</span><span class="n">iteritems</span><span class="p">():</span> <span class="nb">setattr</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">k</span><span class="p">,</span> <span class="n">v</span><span class="p">)</span> <span class="k">def</span> <span class="nf">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span> <span class="sd">"""Return a pair containing the referent and the number of</span> <span class="sd"> times the reference has been called.</span> <span class="sd"> """</span> <span class="n">ob</span> <span class="o">=</span> <span class="nb">super</span><span class="p">(</span><span class="n">ExtendedRef</span><span class="p">,</span> <span class="bp">self</span><span class="p">)</span><span class="o">.</span><span class="fm">__call__</span><span class="p">()</span> <span class="k">if</span> <span class="n">ob</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span> <span class="bp">self</span><span class="o">.</span><span class="n">__counter</span> <span class="o">+=</span> <span class="mi">1</span> <span class="n">ob</span> <span class="o">=</span> <span class="p">(</span><span class="n">ob</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">__counter</span><span class="p">)</span> <span class="k">return</span> <span class="n">ob</span> </pre></div> </div> </div> <div class="section" id="example"> <span id="weakref-example"></span><h2>8.11.2. Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2> <p>This simple example shows how an application can use object IDs to retrieve objects that it has seen before. The IDs of the objects can then be used in other data structures without forcing the objects to remain alive, but the objects can still be retrieved by ID if they do.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">weakref</span> <span class="n">_id2obj_dict</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">WeakValueDictionary</span><span class="p">()</span> <span class="k">def</span> <span class="nf">remember</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span> <span class="n">oid</span> <span class="o">=</span> <span class="nb">id</span><span class="p">(</span><span class="n">obj</span><span class="p">)</span> <span class="n">_id2obj_dict</span><span class="p">[</span><span class="n">oid</span><span class="p">]</span> <span class="o">=</span> <span class="n">obj</span> <span class="k">return</span> <span class="n">oid</span> <span class="k">def</span> <span class="nf">id2obj</span><span class="p">(</span><span class="n">oid</span><span class="p">):</span> <span class="k">return</span> <span class="n">_id2obj_dict</span><span class="p">[</span><span class="n">oid</span><span class="p">]</span> </pre></div> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table of Contents</a></h3> <ul> <li><a class="reference internal" href="#">8.11. <code class="docutils literal notranslate"><span class="pre">weakref</span></code> — Weak references</a><ul> <li><a class="reference internal" href="#weak-reference-objects">8.11.1. Weak Reference Objects</a></li> <li><a class="reference internal" href="#example">8.11.2. Example</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="queue.html" title="previous chapter">8.10. <code class="docutils literal notranslate"><span class="pre">Queue</span></code> — A synchronized queue class</a></p> <h4>Next topic</h4> <p class="topless"><a href="userdict.html" title="next chapter">8.12. <code class="docutils literal notranslate"><span class="pre">UserDict</span></code> — Class wrapper for dictionary objects</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/library/weakref.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="userdict.html" title="8.12. 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