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<div class="section" id="queues">
<h1>18.5.8. Queues<a class="headerlink" href="#queues" title="Permalink to this headline">¶</a></h1>
<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"><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"><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"><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"><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"><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"><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"><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"><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"><span class="pre">0</span></code>, then <code class="docutils literal"><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"><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"><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"><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"><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>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"><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"><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"><span class="pre">True</span></code> if the queue is empty, <code class="docutils literal"><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"><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"><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"><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"><span class="pre">full()</span></code></a> is never <code class="docutils literal"><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>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"><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"><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"><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"><span class="pre">join()</span></code></a> unblocks.</p>
<p>This method is a <a class="reference internal" href="asyncio-task.html#coroutine"><span>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>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"><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"><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"><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"><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"><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"><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"><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"><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"><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"><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"><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"><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"><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"><span class="pre">Queue</span></code></a> object which is full.</p>
</dd></dl>
</div>
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<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>
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<p class="topless"><a href="asyncio-sync.html"
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