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  197.   <section id="queues">

  198. <span id="asyncio-queues"></span><h1>Queues<a class="headerlink" href="#queues" title="Link to this heading">¶</a></h1>

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

  200. <hr class="docutils" />

  201. <p>asyncio queues are designed to be similar to classes of the

  202. <a class="reference internal" href="queue.html#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal notranslate"><span class="pre">queue</span></code></a> module.  Although asyncio queues are not thread-safe,

  203. they are designed to be used specifically in async/await code.</p>

  204. <p>Note that methods of asyncio queues don’t have a <em>timeout</em> parameter;

  205. use <a class="reference internal" href="asyncio-task.html#asyncio.wait_for" title="asyncio.wait_for"><code class="xref py py-func docutils literal notranslate"><span class="pre">asyncio.wait_for()</span></code></a> function to do queue operations with a

  206. timeout.</p>

  207. <p>See also the <a class="reference internal" href="#examples">Examples</a> section below.</p>

  208. <section id="queue">

  209. <h2>Queue<a class="headerlink" href="#queue" title="Link to this heading">¶</a></h2>

  210. <dl class="py class">

  211. <dt class="sig sig-object py" id="asyncio.Queue">

  212. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">Queue</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">maxsize</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">0</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue" title="Link to this definition">¶</a></dt>

  213. <dd><p>A first in, first out (FIFO) queue.</p>

  214. <p>If <em>maxsize</em> is less than or equal to zero, the queue size is

  215. infinite.  If it is an integer greater than <code class="docutils literal notranslate"><span class="pre">0</span></code>, then

  216. <code class="docutils literal notranslate"><span class="pre">await</span> <span class="pre">put()</span></code> blocks when the queue reaches <em>maxsize</em>

  217. until an item is removed by <a class="reference internal" href="#asyncio.Queue.get" title="asyncio.Queue.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code></a>.</p>

  218. <p>Unlike the standard library threading <a class="reference internal" href="queue.html#module-queue" title="queue: A synchronized queue class."><code class="xref py py-mod docutils literal notranslate"><span class="pre">queue</span></code></a>, the size of

  219. the queue is always known and can be returned by calling the

  220. <a class="reference internal" href="#asyncio.Queue.qsize" title="asyncio.Queue.qsize"><code class="xref py py-meth docutils literal notranslate"><span class="pre">qsize()</span></code></a> method.</p>

  221. <div class="versionchanged">

  222. <p><span class="versionmodified changed">Changed in version 3.10: </span>Removed the <em>loop</em> parameter.</p>

  223. </div>

  224. <p>This class is <a class="reference internal" href="asyncio-dev.html#asyncio-multithreading"><span class="std std-ref">not thread safe</span></a>.</p>

  225. <dl class="py attribute">

  226. <dt class="sig sig-object py" id="asyncio.Queue.maxsize">

  227. <span class="sig-name descname"><span class="pre">maxsize</span></span><a class="headerlink" href="#asyncio.Queue.maxsize" title="Link to this definition">¶</a></dt>

  228. <dd><p>Number of items allowed in the queue.</p>

  229. </dd></dl>

  230. 

  231. <dl class="py method">

  232. <dt class="sig sig-object py" id="asyncio.Queue.empty">

  233. <span class="sig-name descname"><span class="pre">empty</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.empty" title="Link to this definition">¶</a></dt>

  234. <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the queue is empty, <code class="docutils literal notranslate"><span class="pre">False</span></code> otherwise.</p>

  235. </dd></dl>

  236. 

  237. <dl class="py method">

  238. <dt class="sig sig-object py" id="asyncio.Queue.full">

  239. <span class="sig-name descname"><span class="pre">full</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.full" title="Link to this definition">¶</a></dt>

  240. <dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if there are <a class="reference internal" href="#asyncio.Queue.maxsize" title="asyncio.Queue.maxsize"><code class="xref py py-attr docutils literal notranslate"><span class="pre">maxsize</span></code></a> items in the queue.</p>

  241. <p>If the queue was initialized with <code class="docutils literal notranslate"><span class="pre">maxsize=0</span></code> (the default),

  242. then <a class="reference internal" href="#asyncio.Queue.full" title="asyncio.Queue.full"><code class="xref py py-meth docutils literal notranslate"><span class="pre">full()</span></code></a> never returns <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>

  243. </dd></dl>

  244. 

  245. <dl class="py method">

  246. <dt class="sig sig-object py" id="asyncio.Queue.get">

  247. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">get</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.get" title="Link to this definition">¶</a></dt>

  248. <dd><p>Remove and return an item from the queue. If queue is empty,

  249. wait until an item is available.</p>

  250. </dd></dl>

  251. 

  252. <dl class="py method">

  253. <dt class="sig sig-object py" id="asyncio.Queue.get_nowait">

  254. <span class="sig-name descname"><span class="pre">get_nowait</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.get_nowait" title="Link to this definition">¶</a></dt>

  255. <dd><p>Return an item if one is immediately available, else raise

  256. <a class="reference internal" href="#asyncio.QueueEmpty" title="asyncio.QueueEmpty"><code class="xref py py-exc docutils literal notranslate"><span class="pre">QueueEmpty</span></code></a>.</p>

  257. </dd></dl>

  258. 

  259. <dl class="py method">

  260. <dt class="sig sig-object py" id="asyncio.Queue.join">

  261. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">join</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.join" title="Link to this definition">¶</a></dt>

  262. <dd><p>Block until all items in the queue have been received and processed.</p>

  263. <p>The count of unfinished tasks goes up whenever an item is added

  264. to the queue. The count goes down whenever a consumer coroutine calls

  265. <a class="reference internal" href="#asyncio.Queue.task_done" title="asyncio.Queue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> to indicate that the item was retrieved and all

  266. work on it is complete.  When the count of unfinished tasks drops

  267. to zero, <a class="reference internal" href="#asyncio.Queue.join" title="asyncio.Queue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> unblocks.</p>

  268. </dd></dl>

  269. 

  270. <dl class="py method">

  271. <dt class="sig sig-object py" id="asyncio.Queue.put">

  272. <em class="property"><span class="pre">coroutine</span> </em><span class="sig-name descname"><span class="pre">put</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">item</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.put" title="Link to this definition">¶</a></dt>

  273. <dd><p>Put an item into the queue. If the queue is full, wait until a

  274. free slot is available before adding the item.</p>

  275. </dd></dl>

  276. 

  277. <dl class="py method">

  278. <dt class="sig sig-object py" id="asyncio.Queue.put_nowait">

  279. <span class="sig-name descname"><span class="pre">put_nowait</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">item</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.put_nowait" title="Link to this definition">¶</a></dt>

  280. <dd><p>Put an item into the queue without blocking.</p>

  281. <p>If no free slot is immediately available, raise <a class="reference internal" href="#asyncio.QueueFull" title="asyncio.QueueFull"><code class="xref py py-exc docutils literal notranslate"><span class="pre">QueueFull</span></code></a>.</p>

  282. </dd></dl>

  283. 

  284. <dl class="py method">

  285. <dt class="sig sig-object py" id="asyncio.Queue.qsize">

  286. <span class="sig-name descname"><span class="pre">qsize</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.qsize" title="Link to this definition">¶</a></dt>

  287. <dd><p>Return the number of items in the queue.</p>

  288. </dd></dl>

  289. 

  290. <dl class="py method">

  291. <dt class="sig sig-object py" id="asyncio.Queue.task_done">

  292. <span class="sig-name descname"><span class="pre">task_done</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.task_done" title="Link to this definition">¶</a></dt>

  293. <dd><p>Indicate that a formerly enqueued task is complete.</p>

  294. <p>Used by queue consumers. For each <a class="reference internal" href="#asyncio.Queue.get" title="asyncio.Queue.get"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get()</span></code></a> used to

  295. fetch a task, a subsequent call to <a class="reference internal" href="#asyncio.Queue.task_done" title="asyncio.Queue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a> tells the

  296. queue that the processing on the task is complete.</p>

  297. <p>If a <a class="reference internal" href="#asyncio.Queue.join" title="asyncio.Queue.join"><code class="xref py py-meth docutils literal notranslate"><span class="pre">join()</span></code></a> is currently blocking, it will resume when all

  298. items have been processed (meaning that a <a class="reference internal" href="#asyncio.Queue.task_done" title="asyncio.Queue.task_done"><code class="xref py py-meth docutils literal notranslate"><span class="pre">task_done()</span></code></a>

  299. call was received for every item that had been <a class="reference internal" href="#asyncio.Queue.put" title="asyncio.Queue.put"><code class="xref py py-meth docutils literal notranslate"><span class="pre">put()</span></code></a>

  300. into the queue).</p>

  301. <p>Raises <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">ValueError</span></code></a> if called more times than there were

  302. items placed in the queue.</p>

  303. </dd></dl>

  304. 

  305. </dd></dl>

  306. 

  307. </section>

  308. <section id="priority-queue">

  309. <h2>Priority Queue<a class="headerlink" href="#priority-queue" title="Link to this heading">¶</a></h2>

  310. <dl class="py class">

  311. <dt class="sig sig-object py" id="asyncio.PriorityQueue">

  312. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">PriorityQueue</span></span><a class="headerlink" href="#asyncio.PriorityQueue" title="Link to this definition">¶</a></dt>

  313. <dd><p>A variant of <a class="reference internal" href="#asyncio.Queue" title="asyncio.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a>; retrieves entries in priority order

  314. (lowest first).</p>

  315. <p>Entries are typically tuples of the form

  316. <code class="docutils literal notranslate"><span class="pre">(priority_number,</span> <span class="pre">data)</span></code>.</p>

  317. </dd></dl>

  318. 

  319. </section>

  320. <section id="lifo-queue">

  321. <h2>LIFO Queue<a class="headerlink" href="#lifo-queue" title="Link to this heading">¶</a></h2>

  322. <dl class="py class">

  323. <dt class="sig sig-object py" id="asyncio.LifoQueue">

  324. <em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">LifoQueue</span></span><a class="headerlink" href="#asyncio.LifoQueue" title="Link to this definition">¶</a></dt>

  325. <dd><p>A variant of <a class="reference internal" href="#asyncio.Queue" title="asyncio.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a> that retrieves most recently added

  326. entries first (last in, first out).</p>

  327. </dd></dl>

  328. 

  329. </section>

  330. <section id="exceptions">

  331. <h2>Exceptions<a class="headerlink" href="#exceptions" title="Link to this heading">¶</a></h2>

  332. <dl class="py exception">

  333. <dt class="sig sig-object py" id="asyncio.QueueEmpty">

  334. <em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">QueueEmpty</span></span><a class="headerlink" href="#asyncio.QueueEmpty" title="Link to this definition">¶</a></dt>

  335. <dd><p>This exception is raised when the <a class="reference internal" href="#asyncio.Queue.get_nowait" title="asyncio.Queue.get_nowait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">get_nowait()</span></code></a> method

  336. is called on an empty queue.</p>

  337. </dd></dl>

  338. 

  339. <dl class="py exception">

  340. <dt class="sig sig-object py" id="asyncio.QueueFull">

  341. <em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">asyncio.</span></span><span class="sig-name descname"><span class="pre">QueueFull</span></span><a class="headerlink" href="#asyncio.QueueFull" title="Link to this definition">¶</a></dt>

  342. <dd><p>Exception raised when the <a class="reference internal" href="#asyncio.Queue.put_nowait" title="asyncio.Queue.put_nowait"><code class="xref py py-meth docutils literal notranslate"><span class="pre">put_nowait()</span></code></a> method is called

  343. on a queue that has reached its <em>maxsize</em>.</p>

  344. </dd></dl>

  345. 

  346. </section>

  347. <section id="examples">

  348. <h2>Examples<a class="headerlink" href="#examples" title="Link to this heading">¶</a></h2>

  349. <p id="asyncio-example-queue-dist">Queues can be used to distribute workload between several

  350. concurrent tasks:</p>

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

  352. <span class="kn">import</span> <span class="nn">random</span>

  353. <span class="kn">import</span> <span class="nn">time</span>

  354. 

  355. 

  356. <span class="k">async</span> <span class="k">def</span> <span class="nf">worker</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">queue</span><span class="p">):</span>

  357.     <span class="k">while</span> <span class="kc">True</span><span class="p">:</span>

  358.         <span class="c1"># Get a &quot;work item&quot; out of the queue.</span>

  359.         <span class="n">sleep_for</span> <span class="o">=</span> <span class="k">await</span> <span class="n">queue</span><span class="o">.</span><span class="n">get</span><span class="p">()</span>

  360. 

  361.         <span class="c1"># Sleep for the &quot;sleep_for&quot; seconds.</span>

  362.         <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="n">sleep_for</span><span class="p">)</span>

  363. 

  364.         <span class="c1"># Notify the queue that the &quot;work item&quot; has been processed.</span>

  365.         <span class="n">queue</span><span class="o">.</span><span class="n">task_done</span><span class="p">()</span>

  366. 

  367.         <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;</span><span class="si">{</span><span class="n">name</span><span class="si">}</span><span class="s1"> has slept for </span><span class="si">{</span><span class="n">sleep_for</span><span class="si">:</span><span class="s1">.2f</span><span class="si">}</span><span class="s1"> seconds&#39;</span><span class="p">)</span>

  368. 

  369. 

  370. <span class="k">async</span> <span class="k">def</span> <span class="nf">main</span><span class="p">():</span>

  371.     <span class="c1"># Create a queue that we will use to store our &quot;workload&quot;.</span>

  372.     <span class="n">queue</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">Queue</span><span class="p">()</span>

  373. 

  374.     <span class="c1"># Generate random timings and put them into the queue.</span>

  375.     <span class="n">total_sleep_time</span> <span class="o">=</span> <span class="mi">0</span>

  376.     <span class="k">for</span> <span class="n">_</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">20</span><span class="p">):</span>

  377.         <span class="n">sleep_for</span> <span class="o">=</span> <span class="n">random</span><span class="o">.</span><span class="n">uniform</span><span class="p">(</span><span class="mf">0.05</span><span class="p">,</span> <span class="mf">1.0</span><span class="p">)</span>

  378.         <span class="n">total_sleep_time</span> <span class="o">+=</span> <span class="n">sleep_for</span>

  379.         <span class="n">queue</span><span class="o">.</span><span class="n">put_nowait</span><span class="p">(</span><span class="n">sleep_for</span><span class="p">)</span>

  380. 

  381.     <span class="c1"># Create three worker tasks to process the queue concurrently.</span>

  382.     <span class="n">tasks</span> <span class="o">=</span> <span class="p">[]</span>

  383.     <span class="k">for</span> <span class="n">i</span> <span class="ow">in</span> <span class="nb">range</span><span class="p">(</span><span class="mi">3</span><span class="p">):</span>

  384.         <span class="n">task</span> <span class="o">=</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">create_task</span><span class="p">(</span><span class="n">worker</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;worker-</span><span class="si">{</span><span class="n">i</span><span class="si">}</span><span class="s1">&#39;</span><span class="p">,</span> <span class="n">queue</span><span class="p">))</span>

  385.         <span class="n">tasks</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="n">task</span><span class="p">)</span>

  386. 

  387.     <span class="c1"># Wait until the queue is fully processed.</span>

  388.     <span class="n">started_at</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">monotonic</span><span class="p">()</span>

  389.     <span class="k">await</span> <span class="n">queue</span><span class="o">.</span><span class="n">join</span><span class="p">()</span>

  390.     <span class="n">total_slept_for</span> <span class="o">=</span> <span class="n">time</span><span class="o">.</span><span class="n">monotonic</span><span class="p">()</span> <span class="o">-</span> <span class="n">started_at</span>

  391. 

  392.     <span class="c1"># Cancel our worker tasks.</span>

  393.     <span class="k">for</span> <span class="n">task</span> <span class="ow">in</span> <span class="n">tasks</span><span class="p">:</span>

  394.         <span class="n">task</span><span class="o">.</span><span class="n">cancel</span><span class="p">()</span>

  395.     <span class="c1"># Wait until all worker tasks are cancelled.</span>

  396.     <span class="k">await</span> <span class="n">asyncio</span><span class="o">.</span><span class="n">gather</span><span class="p">(</span><span class="o">*</span><span class="n">tasks</span><span class="p">,</span> <span class="n">return_exceptions</span><span class="o">=</span><span class="kc">True</span><span class="p">)</span>

  397. 

  398.     <span class="nb">print</span><span class="p">(</span><span class="s1">&#39;====&#39;</span><span class="p">)</span>

  399.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;3 workers slept in parallel for </span><span class="si">{</span><span class="n">total_slept_for</span><span class="si">:</span><span class="s1">.2f</span><span class="si">}</span><span class="s1"> seconds&#39;</span><span class="p">)</span>

  400.     <span class="nb">print</span><span class="p">(</span><span class="sa">f</span><span class="s1">&#39;total expected sleep time: </span><span class="si">{</span><span class="n">total_sleep_time</span><span class="si">:</span><span class="s1">.2f</span><span class="si">}</span><span class="s1"> seconds&#39;</span><span class="p">)</span>

  401. 

  402. 

  403. <span class="n">asyncio</span><span class="o">.</span><span class="n">run</span><span class="p">(</span><span class="n">main</span><span class="p">())</span>

  404. </pre></div>

  405. </div>

  406. </section>

  407. </section>

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  418.     <ul>

  419. <li><a class="reference internal" href="#">Queues</a><ul>

  420. <li><a class="reference internal" href="#queue">Queue</a></li>

  421. <li><a class="reference internal" href="#priority-queue">Priority Queue</a></li>

  422. <li><a class="reference internal" href="#lifo-queue">LIFO Queue</a></li>

  423. <li><a class="reference internal" href="#exceptions">Exceptions</a></li>

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