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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>18.5.8. Queues — Python 3.6.7 documentation</title> <link rel="stylesheet" href="../_static/pydoctheme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 3.6.7 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="18.5.9. Develop with asyncio" href="asyncio-dev.html" /> <link rel="prev" title="18.5.7. Synchronization primitives" href="asyncio-sync.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="https://docs.python.org/3/library/asyncio-queue.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="asyncio-dev.html" title="18.5.9. Develop with asyncio" accesskey="N">next</a> |</li> <li class="right" > <a href="asyncio-sync.html" title="18.5.7. Synchronization primitives" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">3.6.7 Documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" >The Python Standard Library</a> »</li> <li class="nav-item nav-item-2"><a href="ipc.html" >18. Interprocess Communication and Networking</a> »</li> <li class="nav-item nav-item-3"><a href="asyncio.html" accesskey="U">18.5. <code class="docutils literal notranslate"><span class="pre">asyncio</span></code> — Asynchronous I/O, event loop, coroutines and tasks</a> »</li> <li class="right"> <div class="inline-search" style="display: none" role="search"> <form class="inline-search" action="../search.html" method="get"> <input placeholder="Quick search" type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> <script type="text/javascript">$('.inline-search').show(0);</script> | </li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="queues"> <h1>18.5.8. Queues<a class="headerlink" href="#queues" title="Permalink to this headline">¶</a></h1> <p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.6/Lib/asyncio/queues.py">Lib/asyncio/queues.py</a></p> <p>Queues:</p> <ul class="simple"> <li><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></li> <li><a class="reference internal" href="#asyncio.PriorityQueue" title="asyncio.PriorityQueue"><code class="xref py py-class docutils literal notranslate"><span class="pre">PriorityQueue</span></code></a></li> <li><a class="reference internal" href="#asyncio.LifoQueue" title="asyncio.LifoQueue"><code class="xref py py-class docutils literal notranslate"><span class="pre">LifoQueue</span></code></a></li> </ul> <p>asyncio queue API was designed to be close to classes of the <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 (<a class="reference internal" href="queue.html#queue.Queue" title="queue.Queue"><code class="xref py py-class docutils literal notranslate"><span class="pre">Queue</span></code></a>, <a class="reference internal" href="queue.html#queue.PriorityQueue" title="queue.PriorityQueue"><code class="xref py py-class docutils literal notranslate"><span class="pre">PriorityQueue</span></code></a>, <a class="reference internal" href="queue.html#queue.LifoQueue" title="queue.LifoQueue"><code class="xref py py-class docutils literal notranslate"><span class="pre">LifoQueue</span></code></a>), but it has no <em>timeout</em> parameter. The <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 can be used to cancel a task after a timeout.</p> <div class="section" id="queue"> <h2>18.5.8.1. Queue<a class="headerlink" href="#queue" title="Permalink to this headline">¶</a></h2> <dl class="class"> <dt id="asyncio.Queue"> <em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">Queue</code><span class="sig-paren">(</span><em>maxsize=0</em>, <em>*</em>, <em>loop=None</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue" title="Permalink to this definition">¶</a></dt> <dd><p>A queue, useful for coordinating producer and consumer coroutines.</p> <p>If <em>maxsize</em> is less than or equal to zero, the queue size is infinite. If it is an integer greater than <code class="docutils literal notranslate"><span class="pre">0</span></code>, then <code class="docutils literal notranslate"><span class="pre">yield</span> <span class="pre">from</span> <span class="pre">put()</span></code> will block when the queue reaches <em>maxsize</em>, 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> <p>Unlike the standard library <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>, you can reliably know this Queue’s size with <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>, since your single-threaded asyncio application won’t be interrupted between calling <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> and doing an operation on the Queue.</p> <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> <div class="versionchanged"> <p><span class="versionmodified">Changed in version 3.4.4: </span>New <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> and <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> methods.</p> </div> <dl class="method"> <dt id="asyncio.Queue.empty"> <code class="descname">empty</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.empty" title="Permalink to this definition">¶</a></dt> <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> </dd></dl> <dl class="method"> <dt id="asyncio.Queue.full"> <code class="descname">full</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.full" title="Permalink to this definition">¶</a></dt> <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> <div class="admonition note"> <p class="first admonition-title">Note</p> <p class="last">If the Queue was initialized with <code class="docutils literal notranslate"><span class="pre">maxsize=0</span></code> (the default), 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> is never <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p> </div> </dd></dl> <dl class="method"> <dt id="asyncio.Queue.get"> <em class="property">coroutine </em><code class="descname">get</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.get" title="Permalink to this definition">¶</a></dt> <dd><p>Remove and return an item from the queue. If queue is empty, wait until an item is available.</p> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <p class="last">The <a class="reference internal" href="#asyncio.Queue.empty" title="asyncio.Queue.empty"><code class="xref py py-meth docutils literal notranslate"><span class="pre">empty()</span></code></a> method.</p> </div> </dd></dl> <dl class="method"> <dt id="asyncio.Queue.get_nowait"> <code class="descname">get_nowait</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.get_nowait" title="Permalink to this definition">¶</a></dt> <dd><p>Remove and return an item from the queue.</p> <p>Return an item if one is immediately available, else raise <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> </dd></dl> <dl class="method"> <dt id="asyncio.Queue.join"> <em class="property">coroutine </em><code class="descname">join</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.join" title="Permalink to this definition">¶</a></dt> <dd><p>Block until all items in the queue have been gotten and processed.</p> <p>The count of unfinished tasks goes up whenever an item is added to the queue. The count goes down whenever a consumer thread calls <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 work on it is complete. When the count of unfinished tasks drops 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> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.4.</span></p> </div> </dd></dl> <dl class="method"> <dt id="asyncio.Queue.put"> <em class="property">coroutine </em><code class="descname">put</code><span class="sig-paren">(</span><em>item</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.put" title="Permalink to this definition">¶</a></dt> <dd><p>Put an item into the queue. If the queue is full, wait until a free slot is available before adding item.</p> <p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span class="std std-ref">coroutine</span></a>.</p> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <p class="last">The <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> method.</p> </div> </dd></dl> <dl class="method"> <dt id="asyncio.Queue.put_nowait"> <code class="descname">put_nowait</code><span class="sig-paren">(</span><em>item</em><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.put_nowait" title="Permalink to this definition">¶</a></dt> <dd><p>Put an item into the queue without blocking.</p> <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> </dd></dl> <dl class="method"> <dt id="asyncio.Queue.qsize"> <code class="descname">qsize</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.qsize" title="Permalink to this definition">¶</a></dt> <dd><p>Number of items in the queue.</p> </dd></dl> <dl class="method"> <dt id="asyncio.Queue.task_done"> <code class="descname">task_done</code><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#asyncio.Queue.task_done" title="Permalink to this definition">¶</a></dt> <dd><p>Indicate that a formerly enqueued task is complete.</p> <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 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 queue that the processing on the task is complete.</p> <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 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> 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> into the queue).</p> <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 items placed in the queue.</p> <div class="versionadded"> <p><span class="versionmodified">New in version 3.4.4.</span></p> </div> </dd></dl> <dl class="attribute"> <dt id="asyncio.Queue.maxsize"> <code class="descname">maxsize</code><a class="headerlink" href="#asyncio.Queue.maxsize" title="Permalink to this definition">¶</a></dt> <dd><p>Number of items allowed in the queue.</p> </dd></dl> </dd></dl> </div> <div class="section" id="priorityqueue"> <h2>18.5.8.2. PriorityQueue<a class="headerlink" href="#priorityqueue" title="Permalink to this headline">¶</a></h2> <dl class="class"> <dt id="asyncio.PriorityQueue"> <em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">PriorityQueue</code><a class="headerlink" href="#asyncio.PriorityQueue" title="Permalink to this definition">¶</a></dt> <dd><p>A subclass 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 (lowest first).</p> <p>Entries are typically tuples of the form: (priority number, data).</p> </dd></dl> </div> <div class="section" id="lifoqueue"> <h2>18.5.8.3. LifoQueue<a class="headerlink" href="#lifoqueue" title="Permalink to this headline">¶</a></h2> <dl class="class"> <dt id="asyncio.LifoQueue"> <em class="property">class </em><code class="descclassname">asyncio.</code><code class="descname">LifoQueue</code><a class="headerlink" href="#asyncio.LifoQueue" title="Permalink to this definition">¶</a></dt> <dd><p>A subclass 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 entries first.</p> </dd></dl> <div class="section" id="exceptions"> <h3>18.5.8.3.1. Exceptions<a class="headerlink" href="#exceptions" title="Permalink to this headline">¶</a></h3> <dl class="exception"> <dt id="asyncio.QueueEmpty"> <em class="property">exception </em><code class="descclassname">asyncio.</code><code class="descname">QueueEmpty</code><a class="headerlink" href="#asyncio.QueueEmpty" title="Permalink to this definition">¶</a></dt> <dd><p>Exception 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 is called on a <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> object which is empty.</p> </dd></dl> <dl class="exception"> <dt id="asyncio.QueueFull"> <em class="property">exception </em><code class="descclassname">asyncio.</code><code class="descname">QueueFull</code><a class="headerlink" href="#asyncio.QueueFull" title="Permalink to this definition">¶</a></dt> <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 on a <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> object which is full.</p> </dd></dl> </div> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table Of Contents</a></h3> <ul> <li><a class="reference internal" href="#">18.5.8. Queues</a><ul> <li><a class="reference internal" href="#queue">18.5.8.1. Queue</a></li> <li><a class="reference internal" href="#priorityqueue">18.5.8.2. PriorityQueue</a></li> <li><a class="reference internal" href="#lifoqueue">18.5.8.3. LifoQueue</a><ul> <li><a class="reference internal" href="#exceptions">18.5.8.3.1. Exceptions</a></li> </ul> </li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="asyncio-sync.html" title="previous chapter">18.5.7. Synchronization primitives</a></p> <h4>Next topic</h4> <p class="topless"><a href="asyncio-dev.html" title="next chapter">18.5.9. 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