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  102. <li><a class="reference internal" href="#comparing-finalizers-with-del-methods">Comparing finalizers with <code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code> methods</a></li>

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  195.   <section id="module-weakref">

  196. <span id="weakref-weak-references"></span><span id="mod-weakref"></span><h1><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="Link to this heading">¶</a></h1>

  197. <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.12/Lib/weakref.py">Lib/weakref.py</a></p>

  198. <hr class="docutils" />

  199. <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

  200. references</em> to objects.</p>

  201. <p>In the following, the term <em class="dfn">referent</em> means the object which is referred to

  202. by a weak reference.</p>

  203. <p>A weak reference to an object is not enough to keep the object alive: when the

  204. only remaining references to a referent are weak references,

  205. <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

  206. for something else.  However, until the object is actually destroyed the weak

  207. reference may return the object even if there are no strong references to it.</p>

  208. <p>A primary use for weak references is to implement caches or

  209. mappings holding large objects, where it’s desired that a large object not be

  210. kept alive solely because it appears in a cache or mapping.</p>

  211. <p>For example, if you have a number of large binary image objects, you may wish to

  212. associate a name with each.  If you used a Python dictionary to map names to

  213. images, or images to names, the image objects would remain alive just because

  214. they appeared as values or keys in the dictionaries.  The

  215. <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

  216. 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

  217. mappings that don’t keep objects alive solely because they appear in the mapping

  218. objects.  If, for example, an image object is a value in a

  219. <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

  220. image object are the weak references held by weak mappings, garbage collection

  221. can reclaim the object, and its corresponding entries in weak mappings are

  222. simply deleted.</p>

  223. <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

  224. in their implementation, setting up callback functions on the weak references

  225. that notify the weak dictionaries when a key or value has been reclaimed by

  226. garbage collection.  <a class="reference internal" href="#weakref.WeakSet" title="weakref.WeakSet"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakSet</span></code></a> implements the <a class="reference internal" href="stdtypes.html#set" title="set"><code class="xref py py-class docutils literal notranslate"><span class="pre">set</span></code></a> interface,

  227. but keeps weak references to its elements, just like a

  228. <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> does.</p>

  229. <p><a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal notranslate"><span class="pre">finalize</span></code></a> provides a straight forward way to register a

  230. cleanup function to be called when an object is garbage collected.

  231. This is simpler to use than setting up a callback function on a raw

  232. weak reference, since the module automatically ensures that the finalizer

  233. remains alive until the object is collected.</p>

  234. <p>Most programs should find that using one of these weak container types

  235. or <a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal notranslate"><span class="pre">finalize</span></code></a> is all they need – it’s not usually necessary to

  236. create your own weak references directly.  The low-level machinery is

  237. 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>

  238. <p>Not all objects can be weakly referenced. Objects which support weak references

  239. include class instances, functions written in Python (but not in C), instance methods,

  240. sets, frozensets, some <a class="reference internal" href="../glossary.html#term-file-object"><span class="xref std std-term">file objects</span></a>, <a class="reference internal" href="../glossary.html#term-generator"><span class="xref std std-term">generators</span></a>,

  241. type objects, sockets, arrays, deques, regular expression pattern objects, and code

  242. objects.</p>

  243. <div class="versionchanged">

  244. <p><span class="versionmodified changed">Changed in version 3.2: </span>Added support for thread.lock, threading.Lock, and code objects.</p>

  245. </div>

  246. <p>Several built-in types such as <a class="reference internal" href="stdtypes.html#list" title="list"><code class="xref py py-class docutils literal notranslate"><span class="pre">list</span></code></a> 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

  247. support weak references but can add support through subclassing:</p>

  248. <div class="highlight-python3 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>

  249.     <span class="k">pass</span>

  250. 

  251. <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>

  252. </pre></div>

  253. </div>

  254. <div class="impl-detail compound">

  255. <p><strong>CPython implementation detail:</strong> Other built-in types such as <a class="reference internal" href="stdtypes.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#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a> do not support weak

  256. references even when subclassed.</p>

  257. </div>

  258. <p>Extension types can easily be made to support weak references; see

  259. <a class="reference internal" href="../extending/newtypes.html#weakref-support"><span class="std std-ref">Weak Reference Support</span></a>.</p>

  260. <p>When <code class="docutils literal notranslate"><span class="pre">__slots__</span></code> are defined for a given type, weak reference support is

  261. disabled unless a <code class="docutils literal notranslate"><span class="pre">'__weakref__'</span></code> string is also present in the sequence of

  262. strings in the <code class="docutils literal notranslate"><span class="pre">__slots__</span></code> declaration.

  263. See <a class="reference internal" href="../reference/datamodel.html#slots"><span class="std std-ref">__slots__ documentation</span></a> for details.</p>

  264. <dl class="py class">

  265. <dt class="sig sig-object py" id="weakref.ref">

  266. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">ref</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">object</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">callback</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.ref" title="Link to this definition">¶</a></dt>

  267. <dd><p>Return a weak reference to <em>object</em>.  The original object can be retrieved by

  268. calling the reference object if the referent is still alive; if the referent is

  269. 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

  270. 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

  271. weakref object is still alive, the callback will be called when the object is

  272. about to be finalized; the weak reference object will be passed as the only

  273. parameter to the callback; the referent will no longer be available.</p>

  274. <p>It is allowable for many weak references to be constructed for the same object.

  275. Callbacks registered for each weak reference will be called from the most

  276. recently registered callback to the oldest registered callback.</p>

  277. <p>Exceptions raised by the callback will be noted on the standard error output,

  278. but cannot be propagated; they are handled in exactly the same way as exceptions

  279. 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>

  280. <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

  281. maintain their hash value even after the <em>object</em> was deleted.  If

  282. <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,

  283. the call will raise <a class="reference internal" href="exceptions.html#TypeError" title="TypeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">TypeError</span></code></a>.</p>

  284. <p>Weak references support tests for equality, but not ordering.  If the referents

  285. are still alive, two references have the same equality relationship as their

  286. referents (regardless of the <em>callback</em>).  If either referent has been deleted,

  287. the references are equal only if the reference objects are the same object.</p>

  288. <p>This is a subclassable type rather than a factory function.</p>

  289. <dl class="py attribute">

  290. <dt class="sig sig-object py" id="weakref.ref.__callback__">

  291. <span class="sig-name descname"><span class="pre">__callback__</span></span><a class="headerlink" href="#weakref.ref.__callback__" title="Link to this definition">¶</a></dt>

  292. <dd><p>This read-only attribute returns the callback currently associated to the

  293. weakref.  If there is no callback or if the referent of the weakref is

  294. no longer alive then this attribute will have value <code class="docutils literal notranslate"><span class="pre">None</span></code>.</p>

  295. </dd></dl>

  296. 

  297. <div class="versionchanged">

  298. <p><span class="versionmodified changed">Changed in version 3.4: </span>Added the <a class="reference internal" href="#weakref.ref.__callback__" title="weakref.ref.__callback__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">__callback__</span></code></a> attribute.</p>

  299. </div>

  300. </dd></dl>

  301. 

  302. <dl class="py function">

  303. <dt class="sig sig-object py" id="weakref.proxy">

  304. <span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">proxy</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">object</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">callback</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.proxy" title="Link to this definition">¶</a></dt>

  305. <dd><p>Return a proxy to <em>object</em> which uses a weak reference.  This supports use of

  306. the proxy in most contexts instead of requiring the explicit dereferencing used

  307. with weak reference objects.  The returned object will have a type of either

  308. <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

  309. 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

  310. avoids a number of problems related to their fundamentally mutable nature, and

  311. prevents their use as dictionary keys.  <em>callback</em> is the same as the parameter

  312. 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>

  313. <p>Accessing an attribute of the proxy object after the referent is

  314. garbage collected raises <a class="reference internal" href="exceptions.html#ReferenceError" title="ReferenceError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ReferenceError</span></code></a>.</p>

  315. <div class="versionchanged">

  316. <p><span class="versionmodified changed">Changed in version 3.8: </span>Extended the operator support on proxy objects to include the matrix

  317. multiplication operators <code class="docutils literal notranslate"><span class="pre">&#64;</span></code> and <code class="docutils literal notranslate"><span class="pre">&#64;=</span></code>.</p>

  318. </div>

  319. </dd></dl>

  320. 

  321. <dl class="py function">

  322. <dt class="sig sig-object py" id="weakref.getweakrefcount">

  323. <span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">getweakrefcount</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">object</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.getweakrefcount" title="Link to this definition">¶</a></dt>

  324. <dd><p>Return the number of weak references and proxies which refer to <em>object</em>.</p>

  325. </dd></dl>

  326. 

  327. <dl class="py function">

  328. <dt class="sig sig-object py" id="weakref.getweakrefs">

  329. <span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">getweakrefs</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">object</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.getweakrefs" title="Link to this definition">¶</a></dt>

  330. <dd><p>Return a list of all weak reference and proxy objects which refer to <em>object</em>.</p>

  331. </dd></dl>

  332. 

  333. <dl class="py class">

  334. <dt class="sig sig-object py" id="weakref.WeakKeyDictionary">

  335. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">WeakKeyDictionary</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">dict</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakKeyDictionary" title="Link to this definition">¶</a></dt>

  336. <dd><p>Mapping class that references keys weakly.  Entries in the dictionary will be

  337. discarded when there is no longer a strong reference to the key.  This can be

  338. used to associate additional data with an object owned by other parts of an

  339. application without adding attributes to those objects.  This can be especially

  340. useful with objects that override attribute accesses.</p>

  341. <p>Note that when a key with equal value to an existing key (but not equal identity)

  342. is inserted into the dictionary, it replaces the value but does not replace the

  343. existing key. Due to this, when the reference to the original key is deleted, it

  344. also deletes the entry in the dictionary:</p>

  345. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">T</span><span class="p">(</span><span class="nb">str</span><span class="p">):</span> <span class="k">pass</span>

  346. <span class="gp">...</span>

  347. <span class="gp">&gt;&gt;&gt; </span><span class="n">k1</span><span class="p">,</span> <span class="n">k2</span> <span class="o">=</span> <span class="n">T</span><span class="p">(),</span> <span class="n">T</span><span class="p">()</span>

  348. <span class="gp">&gt;&gt;&gt; </span><span class="n">d</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">WeakKeyDictionary</span><span class="p">()</span>

  349. <span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="p">[</span><span class="n">k1</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>   <span class="c1"># d = {k1: 1}</span>

  350. <span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="p">[</span><span class="n">k2</span><span class="p">]</span> <span class="o">=</span> <span class="mi">2</span>   <span class="c1"># d = {k1: 2}</span>

  351. <span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">k1</span>      <span class="c1"># d = {}</span>

  352. </pre></div>

  353. </div>

  354. <p>A workaround would be to remove the key prior to reassignment:</p>

  355. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">T</span><span class="p">(</span><span class="nb">str</span><span class="p">):</span> <span class="k">pass</span>

  356. <span class="gp">...</span>

  357. <span class="gp">&gt;&gt;&gt; </span><span class="n">k1</span><span class="p">,</span> <span class="n">k2</span> <span class="o">=</span> <span class="n">T</span><span class="p">(),</span> <span class="n">T</span><span class="p">()</span>

  358. <span class="gp">&gt;&gt;&gt; </span><span class="n">d</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">WeakKeyDictionary</span><span class="p">()</span>

  359. <span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="p">[</span><span class="n">k1</span><span class="p">]</span> <span class="o">=</span> <span class="mi">1</span>   <span class="c1"># d = {k1: 1}</span>

  360. <span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">d</span><span class="p">[</span><span class="n">k1</span><span class="p">]</span>

  361. <span class="gp">&gt;&gt;&gt; </span><span class="n">d</span><span class="p">[</span><span class="n">k2</span><span class="p">]</span> <span class="o">=</span> <span class="mi">2</span>   <span class="c1"># d = {k2: 2}</span>

  362. <span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">k1</span>      <span class="c1"># d = {k2: 2}</span>

  363. </pre></div>

  364. </div>

  365. <div class="versionchanged">

  366. <p><span class="versionmodified changed">Changed in version 3.9: </span>Added support for <code class="docutils literal notranslate"><span class="pre">|</span></code> and <code class="docutils literal notranslate"><span class="pre">|=</span></code> operators, specified in <span class="target" id="index-0"></span><a class="pep reference external" href="https://peps.python.org/pep-0584/"><strong>PEP 584</strong></a>.</p>

  367. </div>

  368. </dd></dl>

  369. 

  370. <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 an additional method that

  371. exposes the internal references directly.  The references are not guaranteed to

  372. be “live” at the time they are used, so the result of calling the references

  373. needs to be checked before being used.  This can be used to avoid creating

  374. references that will cause the garbage collector to keep the keys around longer

  375. than needed.</p>

  376. <dl class="py method">

  377. <dt class="sig sig-object py" id="weakref.WeakKeyDictionary.keyrefs">

  378. <span class="sig-prename descclassname"><span class="pre">WeakKeyDictionary.</span></span><span class="sig-name descname"><span class="pre">keyrefs</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakKeyDictionary.keyrefs" title="Link to this definition">¶</a></dt>

  379. <dd><p>Return an iterable of the weak references to the keys.</p>

  380. </dd></dl>

  381. 

  382. <dl class="py class">

  383. <dt class="sig sig-object py" id="weakref.WeakValueDictionary">

  384. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">WeakValueDictionary</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">dict</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakValueDictionary" title="Link to this definition">¶</a></dt>

  385. <dd><p>Mapping class that references values weakly.  Entries in the dictionary will be

  386. discarded when no strong reference to the value exists any more.</p>

  387. <div class="versionchanged">

  388. <p><span class="versionmodified changed">Changed in version 3.9: </span>Added support for <code class="docutils literal notranslate"><span class="pre">|</span></code> and <code class="docutils literal notranslate"><span class="pre">|=</span></code> operators, as specified in <span class="target" id="index-1"></span><a class="pep reference external" href="https://peps.python.org/pep-0584/"><strong>PEP 584</strong></a>.</p>

  389. </div>

  390. </dd></dl>

  391. 

  392. <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 an additional method that has the

  393. same issues as the <a class="reference internal" href="#weakref.WeakKeyDictionary.keyrefs" title="weakref.WeakKeyDictionary.keyrefs"><code class="xref py py-meth docutils literal notranslate"><span class="pre">WeakKeyDictionary.keyrefs()</span></code></a> method.</p>

  394. <dl class="py method">

  395. <dt class="sig sig-object py" id="weakref.WeakValueDictionary.valuerefs">

  396. <span class="sig-prename descclassname"><span class="pre">WeakValueDictionary.</span></span><span class="sig-name descname"><span class="pre">valuerefs</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakValueDictionary.valuerefs" title="Link to this definition">¶</a></dt>

  397. <dd><p>Return an iterable of the weak references to the values.</p>

  398. </dd></dl>

  399. 

  400. <dl class="py class">

  401. <dt class="sig sig-object py" id="weakref.WeakSet">

  402. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">WeakSet</span></span><span class="sig-paren">(</span><span class="optional">[</span><em class="sig-param"><span class="n"><span class="pre">elements</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakSet" title="Link to this definition">¶</a></dt>

  403. <dd><p>Set class that keeps weak references to its elements.  An element will be

  404. discarded when no strong reference to it exists any more.</p>

  405. </dd></dl>

  406. 

  407. <dl class="py class">

  408. <dt class="sig sig-object py" id="weakref.WeakMethod">

  409. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">WeakMethod</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">method</span></span></em><span class="optional">[</span>, <em class="sig-param"><span class="n"><span class="pre">callback</span></span></em><span class="optional">]</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.WeakMethod" title="Link to this definition">¶</a></dt>

  410. <dd><p>A custom <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> subclass which simulates a weak reference to a bound

  411. method (i.e., a method defined on a class and looked up on an instance).

  412. Since a bound method is ephemeral, a standard weak reference cannot keep

  413. hold of it.  <a class="reference internal" href="#weakref.WeakMethod" title="weakref.WeakMethod"><code class="xref py py-class docutils literal notranslate"><span class="pre">WeakMethod</span></code></a> has special code to recreate the bound

  414. method until either the object or the original function dies:</p>

  415. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">C</span><span class="p">:</span>

  416. <span class="gp">... </span>    <span class="k">def</span> <span class="nf">method</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  417. <span class="gp">... </span>        <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;method called!&quot;</span><span class="p">)</span>

  418. <span class="gp">...</span>

  419. <span class="gp">&gt;&gt;&gt; </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>

  420. <span class="gp">&gt;&gt;&gt; </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">c</span><span class="o">.</span><span class="n">method</span><span class="p">)</span>

  421. <span class="gp">&gt;&gt;&gt; </span><span class="n">r</span><span class="p">()</span>

  422. <span class="gp">&gt;&gt;&gt; </span><span class="n">r</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">WeakMethod</span><span class="p">(</span><span class="n">c</span><span class="o">.</span><span class="n">method</span><span class="p">)</span>

  423. <span class="gp">&gt;&gt;&gt; </span><span class="n">r</span><span class="p">()</span>

  424. <span class="go">&lt;bound method C.method of &lt;__main__.C object at 0x7fc859830220&gt;&gt;</span>

  425. <span class="gp">&gt;&gt;&gt; </span><span class="n">r</span><span class="p">()()</span>

  426. <span class="go">method called!</span>

  427. <span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">c</span>

  428. <span class="gp">&gt;&gt;&gt; </span><span class="n">gc</span><span class="o">.</span><span class="n">collect</span><span class="p">()</span>

  429. <span class="go">0</span>

  430. <span class="gp">&gt;&gt;&gt; </span><span class="n">r</span><span class="p">()</span>

  431. <span class="gp">&gt;&gt;&gt;</span>

  432. </pre></div>

  433. </div>

  434. <p><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>

  435. <div class="versionadded">

  436. <p><span class="versionmodified added">New in version 3.4.</span></p>

  437. </div>

  438. </dd></dl>

  439. 

  440. <dl class="py class">

  441. <dt class="sig sig-object py" id="weakref.finalize">

  442. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">finalize</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">obj</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">func</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">/</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span><span class="n"><span class="pre">args</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">**</span></span><span class="n"><span class="pre">kwargs</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#weakref.finalize" title="Link to this definition">¶</a></dt>

  443. <dd><p>Return a callable finalizer object which will be called when <em>obj</em>

  444. is garbage collected. Unlike an ordinary weak reference, a finalizer

  445. will always survive until the reference object is collected, greatly

  446. simplifying lifecycle management.</p>

  447. <p>A finalizer is considered <em>alive</em> until it is called (either explicitly

  448. or at garbage collection), and after that it is <em>dead</em>.  Calling a live

  449. finalizer returns the result of evaluating <code class="docutils literal notranslate"><span class="pre">func(*arg,</span> <span class="pre">**kwargs)</span></code>,

  450. whereas calling a dead finalizer 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>

  451. <p>Exceptions raised by finalizer callbacks during garbage collection

  452. will be shown on the standard error output, but cannot be

  453. propagated.  They are handled in the same way as exceptions raised

  454. 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 or a weak reference’s

  455. callback.</p>

  456. <p>When the program exits, each remaining live finalizer is called

  457. unless its <a class="reference internal" href="atexit.html#module-atexit" title="atexit: Register and execute cleanup functions."><code class="xref py py-attr docutils literal notranslate"><span class="pre">atexit</span></code></a> attribute has been set to false.  They

  458. are called in reverse order of creation.</p>

  459. <p>A finalizer will never invoke its callback during the later part of

  460. the <a class="reference internal" href="../glossary.html#term-interpreter-shutdown"><span class="xref std std-term">interpreter shutdown</span></a> when module globals are liable to have

  461. been replaced by <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>

  462. <dl class="py method">

  463. <dt class="sig sig-object py" id="weakref.finalize.__call__">

  464. <span class="sig-name descname"><span class="pre">__call__</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.finalize.__call__" title="Link to this definition">¶</a></dt>

  465. <dd><p>If <em>self</em> is alive then mark it as dead and return the result of

  466. calling <code class="docutils literal notranslate"><span class="pre">func(*args,</span> <span class="pre">**kwargs)</span></code>.  If <em>self</em> is dead then return

  467. <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>

  468. </dd></dl>

  469. 

  470. <dl class="py method">

  471. <dt class="sig sig-object py" id="weakref.finalize.detach">

  472. <span class="sig-name descname"><span class="pre">detach</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.finalize.detach" title="Link to this definition">¶</a></dt>

  473. <dd><p>If <em>self</em> is alive then mark it as dead and return the tuple

  474. <code class="docutils literal notranslate"><span class="pre">(obj,</span> <span class="pre">func,</span> <span class="pre">args,</span> <span class="pre">kwargs)</span></code>.  If <em>self</em> is dead then return

  475. <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>

  476. </dd></dl>

  477. 

  478. <dl class="py method">

  479. <dt class="sig sig-object py" id="weakref.finalize.peek">

  480. <span class="sig-name descname"><span class="pre">peek</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#weakref.finalize.peek" title="Link to this definition">¶</a></dt>

  481. <dd><p>If <em>self</em> is alive then return the tuple <code class="docutils literal notranslate"><span class="pre">(obj,</span> <span class="pre">func,</span> <span class="pre">args,</span>

  482. <span class="pre">kwargs)</span></code>.  If <em>self</em> is dead then return <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>

  483. </dd></dl>

  484. 

  485. <dl class="py attribute">

  486. <dt class="sig sig-object py" id="weakref.finalize.alive">

  487. <span class="sig-name descname"><span class="pre">alive</span></span><a class="headerlink" href="#weakref.finalize.alive" title="Link to this definition">¶</a></dt>

  488. <dd><p>Property which is true if the finalizer is alive, false otherwise.</p>

  489. </dd></dl>

  490. 

  491. <dl class="py attribute">

  492. <dt class="sig sig-object py" id="weakref.finalize.atexit">

  493. <span class="sig-name descname"><span class="pre">atexit</span></span><a class="headerlink" href="#weakref.finalize.atexit" title="Link to this definition">¶</a></dt>

  494. <dd><p>A writable boolean property which by default is true.  When the

  495. program exits, it calls all remaining live finalizers for which

  496. <a class="reference internal" href="#weakref.finalize.atexit" title="weakref.finalize.atexit"><code class="xref py py-attr docutils literal notranslate"><span class="pre">atexit</span></code></a> is true.  They are called in reverse order of

  497. creation.</p>

  498. </dd></dl>

  499. 

  500. <div class="admonition note">

  501. <p class="admonition-title">Note</p>

  502. <p>It is important to ensure that <em>func</em>, <em>args</em> and <em>kwargs</em> do

  503. not own any references to <em>obj</em>, either directly or indirectly,

  504. since otherwise <em>obj</em> will never be garbage collected.  In

  505. particular, <em>func</em> should not be a bound method of <em>obj</em>.</p>

  506. </div>

  507. <div class="versionadded">

  508. <p><span class="versionmodified added">New in version 3.4.</span></p>

  509. </div>

  510. </dd></dl>

  511. 

  512. <dl class="py data">

  513. <dt class="sig sig-object py" id="weakref.ReferenceType">

  514. <span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">ReferenceType</span></span><a class="headerlink" href="#weakref.ReferenceType" title="Link to this definition">¶</a></dt>

  515. <dd><p>The type object for weak references objects.</p>

  516. </dd></dl>

  517. 

  518. <dl class="py data">

  519. <dt class="sig sig-object py" id="weakref.ProxyType">

  520. <span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">ProxyType</span></span><a class="headerlink" href="#weakref.ProxyType" title="Link to this definition">¶</a></dt>

  521. <dd><p>The type object for proxies of objects which are not callable.</p>

  522. </dd></dl>

  523. 

  524. <dl class="py data">

  525. <dt class="sig sig-object py" id="weakref.CallableProxyType">

  526. <span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">CallableProxyType</span></span><a class="headerlink" href="#weakref.CallableProxyType" title="Link to this definition">¶</a></dt>

  527. <dd><p>The type object for proxies of callable objects.</p>

  528. </dd></dl>

  529. 

  530. <dl class="py data">

  531. <dt class="sig sig-object py" id="weakref.ProxyTypes">

  532. <span class="sig-prename descclassname"><span class="pre">weakref.</span></span><span class="sig-name descname"><span class="pre">ProxyTypes</span></span><a class="headerlink" href="#weakref.ProxyTypes" title="Link to this definition">¶</a></dt>

  533. <dd><p>Sequence containing all the type objects for proxies.  This can make it simpler

  534. to test if an object is a proxy without being dependent on naming both proxy

  535. types.</p>

  536. </dd></dl>

  537. 

  538. <div class="admonition seealso">

  539. <p class="admonition-title">See also</p>

  540. <dl class="simple">

  541. <dt><span class="target" id="index-2"></span><a class="pep reference external" href="https://peps.python.org/pep-0205/"><strong>PEP 205</strong></a> - Weak References</dt><dd><p>The proposal and rationale for this feature, including links to earlier

  542. implementations and information about similar features in other languages.</p>

  543. </dd>

  544. </dl>

  545. </div>

  546. <section id="weak-reference-objects">

  547. <span id="weakref-objects"></span><h2>Weak Reference Objects<a class="headerlink" href="#weak-reference-objects" title="Link to this heading">¶</a></h2>

  548. <p>Weak reference objects have no methods and no attributes besides

  549. <a class="reference internal" href="#weakref.ref.__callback__" title="weakref.ref.__callback__"><code class="xref py py-attr docutils literal notranslate"><span class="pre">ref.__callback__</span></code></a>. A weak reference object allows the referent to be

  550. obtained, if it still exists, by calling it:</p>

  551. <div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">weakref</span>

  552. <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">Object</span><span class="p">:</span>

  553. <span class="gp">... </span>    <span class="k">pass</span>

  554. <span class="gp">...</span>

  555. <span class="gp">&gt;&gt;&gt; </span><span class="n">o</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>

  556. <span class="gp">&gt;&gt;&gt; </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>

  557. <span class="gp">&gt;&gt;&gt; </span><span class="n">o2</span> <span class="o">=</span> <span class="n">r</span><span class="p">()</span>

  558. <span class="gp">&gt;&gt;&gt; </span><span class="n">o</span> <span class="ow">is</span> <span class="n">o2</span>

  559. <span class="go">True</span>

  560. </pre></div>

  561. </div>

  562. <p>If the referent no longer exists, calling the reference object returns

  563. <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>

  564. <div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">o</span><span class="p">,</span> <span class="n">o2</span>

  565. <span class="gp">&gt;&gt;&gt; </span><span class="nb">print</span><span class="p">(</span><span class="n">r</span><span class="p">())</span>

  566. <span class="go">None</span>

  567. </pre></div>

  568. </div>

  569. <p>Testing that a weak reference object is still live should be done using the

  570. 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

  571. a reference object should follow this pattern:</p>

  572. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="c1"># r is a weak reference object</span>

  573. <span class="n">o</span> <span class="o">=</span> <span class="n">r</span><span class="p">()</span>

  574. <span class="k">if</span> <span class="n">o</span> <span class="ow">is</span> <span class="kc">None</span><span class="p">:</span>

  575.     <span class="c1"># referent has been garbage collected</span>

  576.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Object has been deallocated; can&#39;t frobnicate.&quot;</span><span class="p">)</span>

  577. <span class="k">else</span><span class="p">:</span>

  578.     <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;Object is still live!&quot;</span><span class="p">)</span>

  579.     <span class="n">o</span><span class="o">.</span><span class="n">do_something_useful</span><span class="p">()</span>

  580. </pre></div>

  581. </div>

  582. <p>Using a separate test for “liveness” creates race conditions in threaded

  583. applications; another thread can cause a weak reference to become invalidated

  584. before the weak reference is called; the idiom shown above is safe in threaded

  585. applications as well as single-threaded applications.</p>

  586. <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.

  587. 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

  588. the memory overhead for each entry in the mapping.  This may be most useful to

  589. associate additional information with a reference, but could also be used to

  590. insert additional processing on calls to retrieve the referent.</p>

  591. <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

  592. additional information about an object and affect the value that’s returned when

  593. the referent is accessed:</p>

  594. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">weakref</span>

  595. 

  596. <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>

  597.     <span class="k">def</span> <span class="fm">__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="p">,</span> <span class="o">**</span><span class="n">annotations</span><span class="p">):</span>

  598.         <span class="nb">super</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>

  599.         <span class="bp">self</span><span class="o">.</span><span class="n">__counter</span> <span class="o">=</span> <span class="mi">0</span>

  600.         <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">items</span><span class="p">():</span>

  601.             <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>

  602. 

  603.     <span class="k">def</span> <span class="fm">__call__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  604. <span class="w">        </span><span class="sd">&quot;&quot;&quot;Return a pair containing the referent and the number of</span>

  605. <span class="sd">        times the reference has been called.</span>

  606. <span class="sd">        &quot;&quot;&quot;</span>

  607.         <span class="n">ob</span> <span class="o">=</span> <span class="nb">super</span><span class="p">()</span><span class="o">.</span><span class="fm">__call__</span><span class="p">()</span>

  608.         <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>

  609.             <span class="bp">self</span><span class="o">.</span><span class="n">__counter</span> <span class="o">+=</span> <span class="mi">1</span>

  610.             <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>

  611.         <span class="k">return</span> <span class="n">ob</span>

  612. </pre></div>

  613. </div>

  614. </section>

  615. <section id="example">

  616. <span id="weakref-example"></span><h2>Example<a class="headerlink" href="#example" title="Link to this heading">¶</a></h2>

  617. <p>This simple example shows how an application can use object IDs to retrieve

  618. objects that it has seen before.  The IDs of the objects can then be used in

  619. other data structures without forcing the objects to remain alive, but the

  620. objects can still be retrieved by ID if they do.</p>

  621. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">weakref</span>

  622. 

  623. <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>

  624. 

  625. <span class="k">def</span> <span class="nf">remember</span><span class="p">(</span><span class="n">obj</span><span class="p">):</span>

  626.     <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>

  627.     <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>

  628.     <span class="k">return</span> <span class="n">oid</span>

  629. 

  630. <span class="k">def</span> <span class="nf">id2obj</span><span class="p">(</span><span class="n">oid</span><span class="p">):</span>

  631.     <span class="k">return</span> <span class="n">_id2obj_dict</span><span class="p">[</span><span class="n">oid</span><span class="p">]</span>

  632. </pre></div>

  633. </div>

  634. </section>

  635. <section id="finalizer-objects">

  636. <span id="finalize-examples"></span><h2>Finalizer Objects<a class="headerlink" href="#finalizer-objects" title="Link to this heading">¶</a></h2>

  637. <p>The main benefit of using <a class="reference internal" href="#weakref.finalize" title="weakref.finalize"><code class="xref py py-class docutils literal notranslate"><span class="pre">finalize</span></code></a> is that it makes it simple

  638. to register a callback without needing to preserve the returned finalizer

  639. object.  For instance</p>

  640. <div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="kn">import</span> <span class="nn">weakref</span>

  641. <span class="gp">&gt;&gt;&gt; </span><span class="k">class</span> <span class="nc">Object</span><span class="p">:</span>

  642. <span class="gp">... </span>    <span class="k">pass</span>

  643. <span class="gp">...</span>

  644. <span class="gp">&gt;&gt;&gt; </span><span class="n">kenny</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>

  645. <span class="gp">&gt;&gt;&gt; </span><span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">kenny</span><span class="p">,</span> <span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;You killed Kenny!&quot;</span><span class="p">)</span>  

  646. <span class="go">&lt;finalize object at ...; for &#39;Object&#39; at ...&gt;</span>

  647. <span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">kenny</span>

  648. <span class="go">You killed Kenny!</span>

  649. </pre></div>

  650. </div>

  651. <p>The finalizer can be called directly as well.  However the finalizer

  652. will invoke the callback at most once.</p>

  653. <div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="k">def</span> <span class="nf">callback</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">z</span><span class="p">):</span>

  654. <span class="gp">... </span>    <span class="nb">print</span><span class="p">(</span><span class="s2">&quot;CALLBACK&quot;</span><span class="p">)</span>

  655. <span class="gp">... </span>    <span class="k">return</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span> <span class="o">+</span> <span class="n">z</span>

  656. <span class="gp">...</span>

  657. <span class="gp">&gt;&gt;&gt; </span><span class="n">obj</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>

  658. <span class="gp">&gt;&gt;&gt; </span><span class="n">f</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">callback</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">z</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>

  659. <span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">f</span><span class="o">.</span><span class="n">alive</span>

  660. <span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">f</span><span class="p">()</span> <span class="o">==</span> <span class="mi">6</span>

  661. <span class="go">CALLBACK</span>

  662. <span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="ow">not</span> <span class="n">f</span><span class="o">.</span><span class="n">alive</span>

  663. <span class="gp">&gt;&gt;&gt; </span><span class="n">f</span><span class="p">()</span>                     <span class="c1"># callback not called because finalizer dead</span>

  664. <span class="gp">&gt;&gt;&gt; </span><span class="k">del</span> <span class="n">obj</span>                 <span class="c1"># callback not called because finalizer dead</span>

  665. </pre></div>

  666. </div>

  667. <p>You can unregister a finalizer using its <a class="reference internal" href="#weakref.finalize.detach" title="weakref.finalize.detach"><code class="xref py py-meth docutils literal notranslate"><span class="pre">detach()</span></code></a>

  668. method.  This kills the finalizer and returns the arguments passed to

  669. the constructor when it was created.</p>

  670. <div class="doctest highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">obj</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>

  671. <span class="gp">&gt;&gt;&gt; </span><span class="n">f</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="n">callback</span><span class="p">,</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">z</span><span class="o">=</span><span class="mi">3</span><span class="p">)</span>

  672. <span class="gp">&gt;&gt;&gt; </span><span class="n">f</span><span class="o">.</span><span class="n">detach</span><span class="p">()</span>                                           

  673. <span class="go">(&lt;...Object object ...&gt;, &lt;function callback ...&gt;, (1, 2), {&#39;z&#39;: 3})</span>

  674. <span class="gp">&gt;&gt;&gt; </span><span class="n">newobj</span><span class="p">,</span> <span class="n">func</span><span class="p">,</span> <span class="n">args</span><span class="p">,</span> <span class="n">kwargs</span> <span class="o">=</span> <span class="n">_</span>

  675. <span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="ow">not</span> <span class="n">f</span><span class="o">.</span><span class="n">alive</span>

  676. <span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">newobj</span> <span class="ow">is</span> <span class="n">obj</span>

  677. <span class="gp">&gt;&gt;&gt; </span><span class="k">assert</span> <span class="n">func</span><span class="p">(</span><span class="o">*</span><span class="n">args</span><span class="p">,</span> <span class="o">**</span><span class="n">kwargs</span><span class="p">)</span> <span class="o">==</span> <span class="mi">6</span>

  678. <span class="go">CALLBACK</span>

  679. </pre></div>

  680. </div>

  681. <p>Unless you set the <a class="reference internal" href="#weakref.finalize.atexit" title="weakref.finalize.atexit"><code class="xref py py-attr docutils literal notranslate"><span class="pre">atexit</span></code></a> attribute to

  682. <a class="reference internal" href="constants.html#False" title="False"><code class="xref py py-const docutils literal notranslate"><span class="pre">False</span></code></a>, a finalizer will be called when the program exits if it

  683. is still alive.  For instance</p>

  684. <div class="highlight-pycon notranslate"><div class="highlight"><pre><span></span><span class="gp">&gt;&gt;&gt; </span><span class="n">obj</span> <span class="o">=</span> <span class="n">Object</span><span class="p">()</span>

  685. <span class="gp">&gt;&gt;&gt; </span><span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">obj</span><span class="p">,</span> <span class="nb">print</span><span class="p">,</span> <span class="s2">&quot;obj dead or exiting&quot;</span><span class="p">)</span>

  686. <span class="go">&lt;finalize object at ...; for &#39;Object&#39; at ...&gt;</span>

  687. <span class="gp">&gt;&gt;&gt; </span><span class="n">exit</span><span class="p">()</span>

  688. <span class="go">obj dead or exiting</span>

  689. </pre></div>

  690. </div>

  691. </section>

  692. <section id="comparing-finalizers-with-del-methods">

  693. <h2>Comparing finalizers with <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> methods<a class="headerlink" href="#comparing-finalizers-with-del-methods" title="Link to this heading">¶</a></h2>

  694. <p>Suppose we want to create a class whose instances represent temporary

  695. directories.  The directories should be deleted with their contents

  696. when the first of the following events occurs:</p>

  697. <ul class="simple">

  698. <li><p>the object is garbage collected,</p></li>

  699. <li><p>the object’s <code class="xref py py-meth docutils literal notranslate"><span class="pre">remove()</span></code> method is called, or</p></li>

  700. <li><p>the program exits.</p></li>

  701. </ul>

  702. <p>We might try to implement the class using a <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 as

  703. follows:</p>

  704. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">TempDir</span><span class="p">:</span>

  705.     <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  706.         <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">tempfile</span><span class="o">.</span><span class="n">mkdtemp</span><span class="p">()</span>

  707. 

  708.     <span class="k">def</span> <span class="nf">remove</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  709.         <span class="k">if</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="ow">is</span> <span class="ow">not</span> <span class="kc">None</span><span class="p">:</span>

  710.             <span class="n">shutil</span><span class="o">.</span><span class="n">rmtree</span><span class="p">(</span><span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>

  711.             <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="kc">None</span>

  712. 

  713.     <span class="nd">@property</span>

  714.     <span class="k">def</span> <span class="nf">removed</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  715.         <span class="k">return</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="ow">is</span> <span class="kc">None</span>

  716. 

  717.     <span class="k">def</span> <span class="fm">__del__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  718.         <span class="bp">self</span><span class="o">.</span><span class="n">remove</span><span class="p">()</span>

  719. </pre></div>

  720. </div>

  721. <p>Starting with Python 3.4, <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> methods no longer prevent

  722. reference cycles from being garbage collected, and module globals are

  723. no longer forced to <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> during <a class="reference internal" href="../glossary.html#term-interpreter-shutdown"><span class="xref std std-term">interpreter shutdown</span></a>.

  724. So this code should work without any issues on CPython.</p>

  725. <p>However, handling of <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> methods is notoriously implementation

  726. specific, since it depends on internal details of the interpreter’s garbage

  727. collector implementation.</p>

  728. <p>A more robust alternative can be to define a finalizer which only references

  729. the specific functions and objects that it needs, rather than having access

  730. to the full state of the object:</p>

  731. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">class</span> <span class="nc">TempDir</span><span class="p">:</span>

  732.     <span class="k">def</span> <span class="fm">__init__</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  733.         <span class="bp">self</span><span class="o">.</span><span class="n">name</span> <span class="o">=</span> <span class="n">tempfile</span><span class="o">.</span><span class="n">mkdtemp</span><span class="p">()</span>

  734.         <span class="bp">self</span><span class="o">.</span><span class="n">_finalizer</span> <span class="o">=</span> <span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="bp">self</span><span class="p">,</span> <span class="n">shutil</span><span class="o">.</span><span class="n">rmtree</span><span class="p">,</span> <span class="bp">self</span><span class="o">.</span><span class="n">name</span><span class="p">)</span>

  735. 

  736.     <span class="k">def</span> <span class="nf">remove</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  737.         <span class="bp">self</span><span class="o">.</span><span class="n">_finalizer</span><span class="p">()</span>

  738. 

  739.     <span class="nd">@property</span>

  740.     <span class="k">def</span> <span class="nf">removed</span><span class="p">(</span><span class="bp">self</span><span class="p">):</span>

  741.         <span class="k">return</span> <span class="ow">not</span> <span class="bp">self</span><span class="o">.</span><span class="n">_finalizer</span><span class="o">.</span><span class="n">alive</span>

  742. </pre></div>

  743. </div>

  744. <p>Defined like this, our finalizer only receives a reference to the details

  745. it needs to clean up the directory appropriately. If the object never gets

  746. garbage collected the finalizer will still be called at exit.</p>

  747. <p>The other advantage of weakref based finalizers is that they can be used to

  748. register finalizers for classes where the definition is controlled by a

  749. third party, such as running code when a module is unloaded:</p>

  750. <div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">weakref</span><span class="o">,</span> <span class="nn">sys</span>

  751. <span class="k">def</span> <span class="nf">unloading_module</span><span class="p">():</span>

  752.     <span class="c1"># implicit reference to the module globals from the function body</span>

  753. <span class="n">weakref</span><span class="o">.</span><span class="n">finalize</span><span class="p">(</span><span class="n">sys</span><span class="o">.</span><span class="n">modules</span><span class="p">[</span><span class="vm">__name__</span><span class="p">],</span> <span class="n">unloading_module</span><span class="p">)</span>

  754. </pre></div>

  755. </div>

  756. <div class="admonition note">

  757. <p class="admonition-title">Note</p>

  758. <p>If you create a finalizer object in a daemonic thread just as the program

  759. exits then there is the possibility that the finalizer

  760. does not get called at exit.  However, in a daemonic thread

  761. <a class="reference internal" href="atexit.html#atexit.register" title="atexit.register"><code class="xref py py-func docutils literal notranslate"><span class="pre">atexit.register()</span></code></a>, <code class="docutils literal notranslate"><span class="pre">try:</span> <span class="pre">...</span> <span class="pre">finally:</span> <span class="pre">...</span></code> and <code class="docutils literal notranslate"><span class="pre">with:</span> <span class="pre">...</span></code>

  762. do not guarantee that cleanup occurs either.</p>

  763. </div>

  764. </section>

  765. </section>

  766. 

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  776.     <ul>

  777. <li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">weakref</span></code> — Weak references</a><ul>

  778. <li><a class="reference internal" href="#weak-reference-objects">Weak Reference Objects</a></li>

  779. <li><a class="reference internal" href="#example">Example</a></li>

  780. <li><a class="reference internal" href="#finalizer-objects">Finalizer Objects</a></li>

  781. <li><a class="reference internal" href="#comparing-finalizers-with-del-methods">Comparing finalizers with <code class="xref py py-meth docutils literal notranslate"><span class="pre">__del__()</span></code> methods</a></li>

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