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<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">concurrent.futures</span></code> — Launching parallel tasks</a><ul>
<li><a class="reference internal" href="#executor-objects">Executor Objects</a></li>
<li><a class="reference internal" href="#threadpoolexecutor">ThreadPoolExecutor</a><ul>
<li><a class="reference internal" href="#threadpoolexecutor-example">ThreadPoolExecutor Example</a></li>
</ul>
</li>
<li><a class="reference internal" href="#processpoolexecutor">ProcessPoolExecutor</a><ul>
<li><a class="reference internal" href="#processpoolexecutor-example">ProcessPoolExecutor Example</a></li>
</ul>
</li>
<li><a class="reference internal" href="#future-objects">Future Objects</a></li>
<li><a class="reference internal" href="#module-functions">Module Functions</a></li>
<li><a class="reference internal" href="#exception-classes">Exception classes</a></li>
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<section id="module-concurrent.futures">
<span id="concurrent-futures-launching-parallel-tasks"></span><h1><a class="reference internal" href="#module-concurrent.futures" title="concurrent.futures: Execute computations concurrently using threads or processes."><code class="xref py py-mod docutils literal notranslate"><span class="pre">concurrent.futures</span></code></a> — Launching parallel tasks<a class="headerlink" href="#module-concurrent.futures" title="Permalink to this headline">¶</a></h1>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.2.</span></p>
</div>
<p><strong>Source code:</strong> <a class="reference external" href="https://github.com/python/cpython/tree/3.10/Lib/concurrent/futures/thread.py">Lib/concurrent/futures/thread.py</a>
and <a class="reference external" href="https://github.com/python/cpython/tree/3.10/Lib/concurrent/futures/process.py">Lib/concurrent/futures/process.py</a></p>
<hr class="docutils" />
<p>The <a class="reference internal" href="#module-concurrent.futures" title="concurrent.futures: Execute computations concurrently using threads or processes."><code class="xref py py-mod docutils literal notranslate"><span class="pre">concurrent.futures</span></code></a> module provides a high-level interface for
asynchronously executing callables.</p>
<p>The asynchronous execution can be performed with threads, using
<a class="reference internal" href="#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadPoolExecutor</span></code></a>, or separate processes, using
<a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProcessPoolExecutor</span></code></a>. Both implement the same interface, which is
defined by the abstract <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> class.</p>
<section id="executor-objects">
<h2>Executor Objects<a class="headerlink" href="#executor-objects" title="Permalink to this headline">¶</a></h2>
<dl class="py class">
<dt class="sig sig-object py" id="concurrent.futures.Executor">
<em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">Executor</span></span><a class="headerlink" href="#concurrent.futures.Executor" title="Permalink to this definition">¶</a></dt>
<dd><p>An abstract class that provides methods to execute calls asynchronously. It
should not be used directly, but through its concrete subclasses.</p>
<blockquote>
<div><dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Executor.submit">
<span class="sig-name descname"><span class="pre">submit</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fn</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="#concurrent.futures.Executor.submit" title="Permalink to this definition">¶</a></dt>
<dd><p>Schedules the callable, <em>fn</em>, to be executed as <code class="docutils literal notranslate"><span class="pre">fn(*args,</span> <span class="pre">**kwargs)</span></code>
and returns a <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> object representing the execution of the
callable.</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">with</span> <span class="n">ThreadPoolExecutor</span><span class="p">(</span><span class="n">max_workers</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span> <span class="k">as</span> <span class="n">executor</span><span class="p">:</span>
<span class="n">future</span> <span class="o">=</span> <span class="n">executor</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="nb">pow</span><span class="p">,</span> <span class="mi">323</span><span class="p">,</span> <span class="mi">1235</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">future</span><span class="o">.</span><span class="n">result</span><span class="p">())</span>
</pre></div>
</div>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Executor.map">
<span class="sig-name descname"><span class="pre">map</span></span><span class="sig-paren">(</span><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><span class="n"><span class="pre">iterables</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">chunksize</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">1</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Executor.map" title="Permalink to this definition">¶</a></dt>
<dd><p>Similar to <a class="reference internal" href="functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate"><span class="pre">map(func,</span> <span class="pre">*iterables)</span></code></a> except:</p>
<ul class="simple">
<li><p>the <em>iterables</em> are collected immediately rather than lazily;</p></li>
<li><p><em>func</em> is executed asynchronously and several calls to
<em>func</em> may be made concurrently.</p></li>
</ul>
<p>The returned iterator raises a <a class="reference internal" href="#concurrent.futures.TimeoutError" title="concurrent.futures.TimeoutError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">concurrent.futures.TimeoutError</span></code></a>
if <a class="reference internal" href="stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a> is called and the result isn’t available
after <em>timeout</em> seconds from the original call to <a class="reference internal" href="#concurrent.futures.Executor.map" title="concurrent.futures.Executor.map"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Executor.map()</span></code></a>.
<em>timeout</em> can be an int or a float. If <em>timeout</em> is not specified or
<code class="docutils literal notranslate"><span class="pre">None</span></code>, there is no limit to the wait time.</p>
<p>If a <em>func</em> call raises an exception, then that exception will be
raised when its value is retrieved from the iterator.</p>
<p>When using <a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProcessPoolExecutor</span></code></a>, this method chops <em>iterables</em>
into a number of chunks which it submits to the pool as separate
tasks. The (approximate) size of these chunks can be specified by
setting <em>chunksize</em> to a positive integer. For very long iterables,
using a large value for <em>chunksize</em> can significantly improve
performance compared to the default size of 1. With
<a class="reference internal" href="#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadPoolExecutor</span></code></a>, <em>chunksize</em> has no effect.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>Added the <em>chunksize</em> argument.</p>
</div>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Executor.shutdown">
<span class="sig-name descname"><span class="pre">shutdown</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">wait</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">True</span></span></em>, <em class="sig-param"><span class="o"><span class="pre">*</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">cancel_futures</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">False</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Executor.shutdown" title="Permalink to this definition">¶</a></dt>
<dd><p>Signal the executor that it should free any resources that it is using
when the currently pending futures are done executing. Calls to
<a class="reference internal" href="#concurrent.futures.Executor.submit" title="concurrent.futures.Executor.submit"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Executor.submit()</span></code></a> and <a class="reference internal" href="#concurrent.futures.Executor.map" title="concurrent.futures.Executor.map"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Executor.map()</span></code></a> made after shutdown will
raise <a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a>.</p>
<p>If <em>wait</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code> then this method will not return until all the
pending futures are done executing and the resources associated with the
executor have been freed. If <em>wait</em> is <code class="docutils literal notranslate"><span class="pre">False</span></code> then this method will
return immediately and the resources associated with the executor will be
freed when all pending futures are done executing. Regardless of the
value of <em>wait</em>, the entire Python program will not exit until all
pending futures are done executing.</p>
<p>If <em>cancel_futures</em> is <code class="docutils literal notranslate"><span class="pre">True</span></code>, this method will cancel all pending
futures that the executor has not started running. Any futures that
are completed or running won’t be cancelled, regardless of the value
of <em>cancel_futures</em>.</p>
<p>If both <em>cancel_futures</em> and <em>wait</em> are <code class="docutils literal notranslate"><span class="pre">True</span></code>, all futures that the
executor has started running will be completed prior to this method
returning. The remaining futures are cancelled.</p>
<p>You can avoid having to call this method explicitly if you use the
<a class="reference internal" href="../reference/compound_stmts.html#with"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">with</span></code></a> statement, which will shutdown the <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a>
(waiting as if <a class="reference internal" href="#concurrent.futures.Executor.shutdown" title="concurrent.futures.Executor.shutdown"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Executor.shutdown()</span></code></a> were called with <em>wait</em> set to
<code class="docutils literal notranslate"><span class="pre">True</span></code>):</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">shutil</span>
<span class="k">with</span> <span class="n">ThreadPoolExecutor</span><span class="p">(</span><span class="n">max_workers</span><span class="o">=</span><span class="mi">4</span><span class="p">)</span> <span class="k">as</span> <span class="n">e</span><span class="p">:</span>
<span class="n">e</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">shutil</span><span class="o">.</span><span class="n">copy</span><span class="p">,</span> <span class="s1">'src1.txt'</span><span class="p">,</span> <span class="s1">'dest1.txt'</span><span class="p">)</span>
<span class="n">e</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">shutil</span><span class="o">.</span><span class="n">copy</span><span class="p">,</span> <span class="s1">'src2.txt'</span><span class="p">,</span> <span class="s1">'dest2.txt'</span><span class="p">)</span>
<span class="n">e</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">shutil</span><span class="o">.</span><span class="n">copy</span><span class="p">,</span> <span class="s1">'src3.txt'</span><span class="p">,</span> <span class="s1">'dest3.txt'</span><span class="p">)</span>
<span class="n">e</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">shutil</span><span class="o">.</span><span class="n">copy</span><span class="p">,</span> <span class="s1">'src4.txt'</span><span class="p">,</span> <span class="s1">'dest4.txt'</span><span class="p">)</span>
</pre></div>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.9: </span>Added <em>cancel_futures</em>.</p>
</div>
</dd></dl>
</div></blockquote>
</dd></dl>
</section>
<section id="threadpoolexecutor">
<h2>ThreadPoolExecutor<a class="headerlink" href="#threadpoolexecutor" title="Permalink to this headline">¶</a></h2>
<p><a class="reference internal" href="#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadPoolExecutor</span></code></a> is an <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> subclass that uses a pool of
threads to execute calls asynchronously.</p>
<p>Deadlocks can occur when the callable associated with a <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> waits on
the results of another <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a>. For example:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">time</span>
<span class="k">def</span> <span class="nf">wait_on_b</span><span class="p">():</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">b</span><span class="o">.</span><span class="n">result</span><span class="p">())</span> <span class="c1"># b will never complete because it is waiting on a.</span>
<span class="k">return</span> <span class="mi">5</span>
<span class="k">def</span> <span class="nf">wait_on_a</span><span class="p">():</span>
<span class="n">time</span><span class="o">.</span><span class="n">sleep</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="nb">print</span><span class="p">(</span><span class="n">a</span><span class="o">.</span><span class="n">result</span><span class="p">())</span> <span class="c1"># a will never complete because it is waiting on b.</span>
<span class="k">return</span> <span class="mi">6</span>
<span class="n">executor</span> <span class="o">=</span> <span class="n">ThreadPoolExecutor</span><span class="p">(</span><span class="n">max_workers</span><span class="o">=</span><span class="mi">2</span><span class="p">)</span>
<span class="n">a</span> <span class="o">=</span> <span class="n">executor</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">wait_on_b</span><span class="p">)</span>
<span class="n">b</span> <span class="o">=</span> <span class="n">executor</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">wait_on_a</span><span class="p">)</span>
</pre></div>
</div>
<p>And:</p>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">wait_on_future</span><span class="p">():</span>
<span class="n">f</span> <span class="o">=</span> <span class="n">executor</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="nb">pow</span><span class="p">,</span> <span class="mi">5</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="c1"># This will never complete because there is only one worker thread and</span>
<span class="c1"># it is executing this function.</span>
<span class="nb">print</span><span class="p">(</span><span class="n">f</span><span class="o">.</span><span class="n">result</span><span class="p">())</span>
<span class="n">executor</span> <span class="o">=</span> <span class="n">ThreadPoolExecutor</span><span class="p">(</span><span class="n">max_workers</span><span class="o">=</span><span class="mi">1</span><span class="p">)</span>
<span class="n">executor</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">wait_on_future</span><span class="p">)</span>
</pre></div>
</div>
<dl class="py class">
<dt class="sig sig-object py" id="concurrent.futures.ThreadPoolExecutor">
<em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">ThreadPoolExecutor</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">max_workers</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">thread_name_prefix</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">''</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">initializer</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">initargs</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">()</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.ThreadPoolExecutor" title="Permalink to this definition">¶</a></dt>
<dd><p>An <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> subclass that uses a pool of at most <em>max_workers</em>
threads to execute calls asynchronously.</p>
<p>All threads enqueued to <code class="docutils literal notranslate"><span class="pre">ThreadPoolExecutor</span></code> will be joined before the
interpreter can exit. Note that the exit handler which does this is
executed <em>before</em> any exit handlers added using <code class="docutils literal notranslate"><span class="pre">atexit</span></code>. This means
exceptions in the main thread must be caught and handled in order to
signal threads to exit gracefully. For this reason, it is recommended
that <code class="docutils literal notranslate"><span class="pre">ThreadPoolExecutor</span></code> not be used for long-running tasks.</p>
<p><em>initializer</em> is an optional callable that is called at the start of
each worker thread; <em>initargs</em> is a tuple of arguments passed to the
initializer. Should <em>initializer</em> raise an exception, all currently
pending jobs will raise a <a class="reference internal" href="#concurrent.futures.thread.BrokenThreadPool" title="concurrent.futures.thread.BrokenThreadPool"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BrokenThreadPool</span></code></a>,
as well as any attempt to submit more jobs to the pool.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.5: </span>If <em>max_workers</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> or
not given, it will default to the number of processors on the machine,
multiplied by <code class="docutils literal notranslate"><span class="pre">5</span></code>, assuming that <a class="reference internal" href="#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadPoolExecutor</span></code></a> is often
used to overlap I/O instead of CPU work and the number of workers
should be higher than the number of workers
for <a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProcessPoolExecutor</span></code></a>.</p>
</div>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.6: </span>The <em>thread_name_prefix</em> argument was added to allow users to
control the <a class="reference internal" href="threading.html#threading.Thread" title="threading.Thread"><code class="xref py py-class docutils literal notranslate"><span class="pre">threading.Thread</span></code></a> names for worker threads created by
the pool for easier debugging.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.7: </span>Added the <em>initializer</em> and <em>initargs</em> arguments.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.8: </span>Default value of <em>max_workers</em> is changed to <code class="docutils literal notranslate"><span class="pre">min(32,</span> <span class="pre">os.cpu_count()</span> <span class="pre">+</span> <span class="pre">4)</span></code>.
This default value preserves at least 5 workers for I/O bound tasks.
It utilizes at most 32 CPU cores for CPU bound tasks which release the GIL.
And it avoids using very large resources implicitly on many-core machines.</p>
<p>ThreadPoolExecutor now reuses idle worker threads before starting
<em>max_workers</em> worker threads too.</p>
</div>
</dd></dl>
<section id="threadpoolexecutor-example">
<span id="id1"></span><h3>ThreadPoolExecutor Example<a class="headerlink" href="#threadpoolexecutor-example" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">concurrent.futures</span>
<span class="kn">import</span> <span class="nn">urllib.request</span>
<span class="n">URLS</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'http://www.foxnews.com/'</span><span class="p">,</span>
<span class="s1">'http://www.cnn.com/'</span><span class="p">,</span>
<span class="s1">'http://europe.wsj.com/'</span><span class="p">,</span>
<span class="s1">'http://www.bbc.co.uk/'</span><span class="p">,</span>
<span class="s1">'http://nonexistant-subdomain.python.org/'</span><span class="p">]</span>
<span class="c1"># Retrieve a single page and report the URL and contents</span>
<span class="k">def</span> <span class="nf">load_url</span><span class="p">(</span><span class="n">url</span><span class="p">,</span> <span class="n">timeout</span><span class="p">):</span>
<span class="k">with</span> <span class="n">urllib</span><span class="o">.</span><span class="n">request</span><span class="o">.</span><span class="n">urlopen</span><span class="p">(</span><span class="n">url</span><span class="p">,</span> <span class="n">timeout</span><span class="o">=</span><span class="n">timeout</span><span class="p">)</span> <span class="k">as</span> <span class="n">conn</span><span class="p">:</span>
<span class="k">return</span> <span class="n">conn</span><span class="o">.</span><span class="n">read</span><span class="p">()</span>
<span class="c1"># We can use a with statement to ensure threads are cleaned up promptly</span>
<span class="k">with</span> <span class="n">concurrent</span><span class="o">.</span><span class="n">futures</span><span class="o">.</span><span class="n">ThreadPoolExecutor</span><span class="p">(</span><span class="n">max_workers</span><span class="o">=</span><span class="mi">5</span><span class="p">)</span> <span class="k">as</span> <span class="n">executor</span><span class="p">:</span>
<span class="c1"># Start the load operations and mark each future with its URL</span>
<span class="n">future_to_url</span> <span class="o">=</span> <span class="p">{</span><span class="n">executor</span><span class="o">.</span><span class="n">submit</span><span class="p">(</span><span class="n">load_url</span><span class="p">,</span> <span class="n">url</span><span class="p">,</span> <span class="mi">60</span><span class="p">):</span> <span class="n">url</span> <span class="k">for</span> <span class="n">url</span> <span class="ow">in</span> <span class="n">URLS</span><span class="p">}</span>
<span class="k">for</span> <span class="n">future</span> <span class="ow">in</span> <span class="n">concurrent</span><span class="o">.</span><span class="n">futures</span><span class="o">.</span><span class="n">as_completed</span><span class="p">(</span><span class="n">future_to_url</span><span class="p">):</span>
<span class="n">url</span> <span class="o">=</span> <span class="n">future_to_url</span><span class="p">[</span><span class="n">future</span><span class="p">]</span>
<span class="k">try</span><span class="p">:</span>
<span class="n">data</span> <span class="o">=</span> <span class="n">future</span><span class="o">.</span><span class="n">result</span><span class="p">()</span>
<span class="k">except</span> <span class="ne">Exception</span> <span class="k">as</span> <span class="n">exc</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">%r</span><span class="s1"> generated an exception: </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">url</span><span class="p">,</span> <span class="n">exc</span><span class="p">))</span>
<span class="k">else</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">%r</span><span class="s1"> page is </span><span class="si">%d</span><span class="s1"> bytes'</span> <span class="o">%</span> <span class="p">(</span><span class="n">url</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="n">data</span><span class="p">)))</span>
</pre></div>
</div>
</section>
</section>
<section id="processpoolexecutor">
<h2>ProcessPoolExecutor<a class="headerlink" href="#processpoolexecutor" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProcessPoolExecutor</span></code></a> class is an <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> subclass that
uses a pool of processes to execute calls asynchronously.
<a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProcessPoolExecutor</span></code></a> uses the <a class="reference internal" href="multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><code class="xref py py-mod docutils literal notranslate"><span class="pre">multiprocessing</span></code></a> module, which
allows it to side-step the <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><span class="xref std std-term">Global Interpreter Lock</span></a> but also means that
only picklable objects can be executed and returned.</p>
<p>The <code class="docutils literal notranslate"><span class="pre">__main__</span></code> module must be importable by worker subprocesses. This means
that <a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProcessPoolExecutor</span></code></a> will not work in the interactive interpreter.</p>
<p>Calling <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> or <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> methods from a callable submitted
to a <a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate"><span class="pre">ProcessPoolExecutor</span></code></a> will result in deadlock.</p>
<dl class="py class">
<dt class="sig sig-object py" id="concurrent.futures.ProcessPoolExecutor">
<em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">ProcessPoolExecutor</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">max_workers</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">mp_context</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">initializer</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">initargs</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">()</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.ProcessPoolExecutor" title="Permalink to this definition">¶</a></dt>
<dd><p>An <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> subclass that executes calls asynchronously using a pool
of at most <em>max_workers</em> processes. If <em>max_workers</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> or not
given, it will default to the number of processors on the machine.
If <em>max_workers</em> is less than or equal to <code class="docutils literal notranslate"><span class="pre">0</span></code>, then a <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>
will be raised.
On Windows, <em>max_workers</em> must be less than or equal to <code class="docutils literal notranslate"><span class="pre">61</span></code>. If it is not
then <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> will be raised. If <em>max_workers</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code>, then
the default chosen will be at most <code class="docutils literal notranslate"><span class="pre">61</span></code>, even if more processors are
available.
<em>mp_context</em> can be a multiprocessing context or None. It will be used to
launch the workers. If <em>mp_context</em> is <code class="docutils literal notranslate"><span class="pre">None</span></code> or not given, the default
multiprocessing context is used.</p>
<p><em>initializer</em> is an optional callable that is called at the start of
each worker process; <em>initargs</em> is a tuple of arguments passed to the
initializer. Should <em>initializer</em> raise an exception, all currently
pending jobs will raise a <a class="reference internal" href="#concurrent.futures.process.BrokenProcessPool" title="concurrent.futures.process.BrokenProcessPool"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BrokenProcessPool</span></code></a>,
as well as any attempt to submit more jobs to the pool.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.3: </span>When one of the worker processes terminates abruptly, a
<code class="xref py py-exc docutils literal notranslate"><span class="pre">BrokenProcessPool</span></code> error is now raised. Previously, behaviour
was undefined but operations on the executor or its futures would often
freeze or deadlock.</p>
</div>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.7: </span>The <em>mp_context</em> argument was added to allow users to control the
start_method for worker processes created by the pool.</p>
<p>Added the <em>initializer</em> and <em>initargs</em> arguments.</p>
</div>
</dd></dl>
<section id="processpoolexecutor-example">
<span id="id2"></span><h3>ProcessPoolExecutor Example<a class="headerlink" href="#processpoolexecutor-example" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python3 notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">concurrent.futures</span>
<span class="kn">import</span> <span class="nn">math</span>
<span class="n">PRIMES</span> <span class="o">=</span> <span class="p">[</span>
<span class="mi">112272535095293</span><span class="p">,</span>
<span class="mi">112582705942171</span><span class="p">,</span>
<span class="mi">112272535095293</span><span class="p">,</span>
<span class="mi">115280095190773</span><span class="p">,</span>
<span class="mi">115797848077099</span><span class="p">,</span>
<span class="mi">1099726899285419</span><span class="p">]</span>
<span class="k">def</span> <span class="nf">is_prime</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<span class="k">if</span> <span class="n">n</span> <span class="o"><</span> <span class="mi">2</span><span class="p">:</span>
<span class="k">return</span> <span class="kc">False</span>
<span class="k">if</span> <span class="n">n</span> <span class="o">==</span> <span class="mi">2</span><span class="p">:</span>
<span class="k">return</span> <span class="kc">True</span>
<span class="k">if</span> <span class="n">n</span> <span class="o">%</span> <span class="mi">2</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">return</span> <span class="kc">False</span>
<span class="n">sqrt_n</span> <span class="o">=</span> <span class="nb">int</span><span class="p">(</span><span class="n">math</span><span class="o">.</span><span class="n">floor</span><span class="p">(</span><span class="n">math</span><span class="o">.</span><span class="n">sqrt</span><span class="p">(</span><span class="n">n</span><span class="p">)))</span>
<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> <span class="n">sqrt_n</span> <span class="o">+</span> <span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">):</span>
<span class="k">if</span> <span class="n">n</span> <span class="o">%</span> <span class="n">i</span> <span class="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">return</span> <span class="kc">False</span>
<span class="k">return</span> <span class="kc">True</span>
<span class="k">def</span> <span class="nf">main</span><span class="p">():</span>
<span class="k">with</span> <span class="n">concurrent</span><span class="o">.</span><span class="n">futures</span><span class="o">.</span><span class="n">ProcessPoolExecutor</span><span class="p">()</span> <span class="k">as</span> <span class="n">executor</span><span class="p">:</span>
<span class="k">for</span> <span class="n">number</span><span class="p">,</span> <span class="n">prime</span> <span class="ow">in</span> <span class="nb">zip</span><span class="p">(</span><span class="n">PRIMES</span><span class="p">,</span> <span class="n">executor</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="n">is_prime</span><span class="p">,</span> <span class="n">PRIMES</span><span class="p">)):</span>
<span class="nb">print</span><span class="p">(</span><span class="s1">'</span><span class="si">%d</span><span class="s1"> is prime: </span><span class="si">%s</span><span class="s1">'</span> <span class="o">%</span> <span class="p">(</span><span class="n">number</span><span class="p">,</span> <span class="n">prime</span><span class="p">))</span>
<span class="k">if</span> <span class="vm">__name__</span> <span class="o">==</span> <span class="s1">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
</section>
</section>
<section id="future-objects">
<h2>Future Objects<a class="headerlink" href="#future-objects" title="Permalink to this headline">¶</a></h2>
<p>The <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> class encapsulates the asynchronous execution of a callable.
<a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> instances are created by <a class="reference internal" href="#concurrent.futures.Executor.submit" title="concurrent.futures.Executor.submit"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Executor.submit()</span></code></a>.</p>
<dl class="py class">
<dt class="sig sig-object py" id="concurrent.futures.Future">
<em class="property"><span class="pre">class</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">Future</span></span><a class="headerlink" href="#concurrent.futures.Future" title="Permalink to this definition">¶</a></dt>
<dd><p>Encapsulates the asynchronous execution of a callable. <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a>
instances are created by <a class="reference internal" href="#concurrent.futures.Executor.submit" title="concurrent.futures.Executor.submit"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Executor.submit()</span></code></a> and should not be created
directly except for testing.</p>
<blockquote>
<div><dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.cancel">
<span class="sig-name descname"><span class="pre">cancel</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.cancel" title="Permalink to this definition">¶</a></dt>
<dd><p>Attempt to cancel the call. If the call is currently being executed or
finished running and cannot be cancelled then the method will return
<code class="docutils literal notranslate"><span class="pre">False</span></code>, otherwise the call will be cancelled and the method will
return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.cancelled">
<span class="sig-name descname"><span class="pre">cancelled</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.cancelled" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the call was successfully cancelled.</p>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.running">
<span class="sig-name descname"><span class="pre">running</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.running" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the call is currently being executed and cannot be
cancelled.</p>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.done">
<span class="sig-name descname"><span class="pre">done</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.done" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <code class="docutils literal notranslate"><span class="pre">True</span></code> if the call was successfully cancelled or finished
running.</p>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.result">
<span class="sig-name descname"><span class="pre">result</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.result" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the value returned by the call. If the call hasn’t yet completed
then this method will wait up to <em>timeout</em> seconds. If the call hasn’t
completed in <em>timeout</em> seconds, then a
<a class="reference internal" href="#concurrent.futures.TimeoutError" title="concurrent.futures.TimeoutError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">concurrent.futures.TimeoutError</span></code></a> will be raised. <em>timeout</em> can be
an int or float. If <em>timeout</em> is not specified or <code class="docutils literal notranslate"><span class="pre">None</span></code>, there is no
limit to the wait time.</p>
<p>If the future is cancelled before completing then <a class="reference internal" href="#concurrent.futures.CancelledError" title="concurrent.futures.CancelledError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">CancelledError</span></code></a>
will be raised.</p>
<p>If the call raised an exception, this method will raise the same exception.</p>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.exception">
<span class="sig-name descname"><span class="pre">exception</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.exception" title="Permalink to this definition">¶</a></dt>
<dd><p>Return the exception raised by the call. If the call hasn’t yet
completed then this method will wait up to <em>timeout</em> seconds. If the
call hasn’t completed in <em>timeout</em> seconds, then a
<a class="reference internal" href="#concurrent.futures.TimeoutError" title="concurrent.futures.TimeoutError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">concurrent.futures.TimeoutError</span></code></a> will be raised. <em>timeout</em> can be
an int or float. If <em>timeout</em> is not specified or <code class="docutils literal notranslate"><span class="pre">None</span></code>, there is no
limit to the wait time.</p>
<p>If the future is cancelled before completing then <a class="reference internal" href="#concurrent.futures.CancelledError" title="concurrent.futures.CancelledError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">CancelledError</span></code></a>
will be raised.</p>
<p>If the call completed without raising, <code class="docutils literal notranslate"><span class="pre">None</span></code> is returned.</p>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.add_done_callback">
<span class="sig-name descname"><span class="pre">add_done_callback</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fn</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.add_done_callback" title="Permalink to this definition">¶</a></dt>
<dd><p>Attaches the callable <em>fn</em> to the future. <em>fn</em> will be called, with the
future as its only argument, when the future is cancelled or finishes
running.</p>
<p>Added callables are called in the order that they were added and are
always called in a thread belonging to the process that added them. If
the callable raises an <a class="reference internal" href="exceptions.html#Exception" title="Exception"><code class="xref py py-exc docutils literal notranslate"><span class="pre">Exception</span></code></a> subclass, it will be logged and
ignored. If the callable raises a <a class="reference internal" href="exceptions.html#BaseException" title="BaseException"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BaseException</span></code></a> subclass, the
behavior is undefined.</p>
<p>If the future has already completed or been cancelled, <em>fn</em> will be
called immediately.</p>
</dd></dl>
</div></blockquote>
<p>The following <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> methods are meant for use in unit tests and
<a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> implementations.</p>
<blockquote>
<div><dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.set_running_or_notify_cancel">
<span class="sig-name descname"><span class="pre">set_running_or_notify_cancel</span></span><span class="sig-paren">(</span><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.set_running_or_notify_cancel" title="Permalink to this definition">¶</a></dt>
<dd><p>This method should only be called by <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> implementations
before executing the work associated with the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> and by unit
tests.</p>
<p>If the method returns <code class="docutils literal notranslate"><span class="pre">False</span></code> then the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> was cancelled,
i.e. <a class="reference internal" href="#concurrent.futures.Future.cancel" title="concurrent.futures.Future.cancel"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Future.cancel()</span></code></a> was called and returned <code class="docutils literal notranslate"><span class="pre">True</span></code>. Any threads
waiting on the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> completing (i.e. through
<a class="reference internal" href="#concurrent.futures.as_completed" title="concurrent.futures.as_completed"><code class="xref py py-func docutils literal notranslate"><span class="pre">as_completed()</span></code></a> or <a class="reference internal" href="#concurrent.futures.wait" title="concurrent.futures.wait"><code class="xref py py-func docutils literal notranslate"><span class="pre">wait()</span></code></a>) will be woken up.</p>
<p>If the method returns <code class="docutils literal notranslate"><span class="pre">True</span></code> then the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> was not cancelled
and has been put in the running state, i.e. calls to
<a class="reference internal" href="#concurrent.futures.Future.running" title="concurrent.futures.Future.running"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Future.running()</span></code></a> will return <code class="docutils literal notranslate"><span class="pre">True</span></code>.</p>
<p>This method can only be called once and cannot be called after
<a class="reference internal" href="#concurrent.futures.Future.set_result" title="concurrent.futures.Future.set_result"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Future.set_result()</span></code></a> or <a class="reference internal" href="#concurrent.futures.Future.set_exception" title="concurrent.futures.Future.set_exception"><code class="xref py py-meth docutils literal notranslate"><span class="pre">Future.set_exception()</span></code></a> have been
called.</p>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.set_result">
<span class="sig-name descname"><span class="pre">set_result</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">result</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.set_result" title="Permalink to this definition">¶</a></dt>
<dd><p>Sets the result of the work associated with the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> to
<em>result</em>.</p>
<p>This method should only be used by <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> implementations and
unit tests.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.8: </span>This method raises
<a class="reference internal" href="#concurrent.futures.InvalidStateError" title="concurrent.futures.InvalidStateError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">concurrent.futures.InvalidStateError</span></code></a> if the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> is
already done.</p>
</div>
</dd></dl>
<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.set_exception">
<span class="sig-name descname"><span class="pre">set_exception</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">exception</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.Future.set_exception" title="Permalink to this definition">¶</a></dt>
<dd><p>Sets the result of the work associated with the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> to the
<a class="reference internal" href="exceptions.html#Exception" title="Exception"><code class="xref py py-class docutils literal notranslate"><span class="pre">Exception</span></code></a> <em>exception</em>.</p>
<p>This method should only be used by <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> implementations and
unit tests.</p>
<div class="versionchanged">
<p><span class="versionmodified changed">Changed in version 3.8: </span>This method raises
<a class="reference internal" href="#concurrent.futures.InvalidStateError" title="concurrent.futures.InvalidStateError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">concurrent.futures.InvalidStateError</span></code></a> if the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> is
already done.</p>
</div>
</dd></dl>
</div></blockquote>
</dd></dl>
</section>
<section id="module-functions">
<h2>Module Functions<a class="headerlink" href="#module-functions" title="Permalink to this headline">¶</a></h2>
<dl class="py function">
<dt class="sig sig-object py" id="concurrent.futures.wait">
<span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">wait</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fs</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">return_when</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">ALL_COMPLETED</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.wait" title="Permalink to this definition">¶</a></dt>
<dd><p>Wait for the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> instances (possibly created by different
<a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> instances) given by <em>fs</em> to complete. Duplicate futures
given to <em>fs</em> are removed and will be returned only once. Returns a named
2-tuple of sets. The first set, named <code class="docutils literal notranslate"><span class="pre">done</span></code>, contains the futures that
completed (finished or cancelled futures) before the wait completed. The
second set, named <code class="docutils literal notranslate"><span class="pre">not_done</span></code>, contains the futures that did not complete
(pending or running futures).</p>
<p><em>timeout</em> can be used to control the maximum number of seconds to wait before
returning. <em>timeout</em> can be an int or float. If <em>timeout</em> is not specified
or <code class="docutils literal notranslate"><span class="pre">None</span></code>, there is no limit to the wait time.</p>
<p><em>return_when</em> indicates when this function should return. It must be one of
the following constants:</p>
<table class="docutils align-default">
<colgroup>
<col style="width: 42%" />
<col style="width: 58%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head"><p>Constant</p></th>
<th class="head"><p>Description</p></th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">FIRST_COMPLETED</span></code></p></td>
<td><p>The function will return when any
future finishes or is cancelled.</p></td>
</tr>
<tr class="row-odd"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">FIRST_EXCEPTION</span></code></p></td>
<td><p>The function will return when any
future finishes by raising an
exception. If no future raises an
exception then it is equivalent to
<code class="xref py py-const docutils literal notranslate"><span class="pre">ALL_COMPLETED</span></code>.</p></td>
</tr>
<tr class="row-even"><td><p><code class="xref py py-const docutils literal notranslate"><span class="pre">ALL_COMPLETED</span></code></p></td>
<td><p>The function will return when all
futures finish or are cancelled.</p></td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="py function">
<dt class="sig sig-object py" id="concurrent.futures.as_completed">
<span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">as_completed</span></span><span class="sig-paren">(</span><em class="sig-param"><span class="n"><span class="pre">fs</span></span></em>, <em class="sig-param"><span class="n"><span class="pre">timeout</span></span><span class="o"><span class="pre">=</span></span><span class="default_value"><span class="pre">None</span></span></em><span class="sig-paren">)</span><a class="headerlink" href="#concurrent.futures.as_completed" title="Permalink to this definition">¶</a></dt>
<dd><p>Returns an iterator over the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate"><span class="pre">Future</span></code></a> instances (possibly created by
different <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate"><span class="pre">Executor</span></code></a> instances) given by <em>fs</em> that yields futures as
they complete (finished or cancelled futures). Any futures given by <em>fs</em> that
are duplicated will be returned once. Any futures that completed before
<a class="reference internal" href="#concurrent.futures.as_completed" title="concurrent.futures.as_completed"><code class="xref py py-func docutils literal notranslate"><span class="pre">as_completed()</span></code></a> is called will be yielded first. The returned iterator
raises a <a class="reference internal" href="#concurrent.futures.TimeoutError" title="concurrent.futures.TimeoutError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">concurrent.futures.TimeoutError</span></code></a> if <a class="reference internal" href="stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate"><span class="pre">__next__()</span></code></a>
is called and the result isn’t available after <em>timeout</em> seconds from the
original call to <a class="reference internal" href="#concurrent.futures.as_completed" title="concurrent.futures.as_completed"><code class="xref py py-func docutils literal notranslate"><span class="pre">as_completed()</span></code></a>. <em>timeout</em> can be an int or float. If
<em>timeout</em> is not specified or <code class="docutils literal notranslate"><span class="pre">None</span></code>, there is no limit to the wait time.</p>
</dd></dl>
<div class="admonition seealso">
<p class="admonition-title">See also</p>
<dl class="simple">
<dt><span class="target" id="index-0"></span><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3148"><strong>PEP 3148</strong></a> – futures - execute computations asynchronously</dt><dd><p>The proposal which described this feature for inclusion in the Python
standard library.</p>
</dd>
</dl>
</div>
</section>
<section id="exception-classes">
<h2>Exception classes<a class="headerlink" href="#exception-classes" title="Permalink to this headline">¶</a></h2>
<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.CancelledError">
<em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">CancelledError</span></span><a class="headerlink" href="#concurrent.futures.CancelledError" title="Permalink to this definition">¶</a></dt>
<dd><p>Raised when a future is cancelled.</p>
</dd></dl>
<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.TimeoutError">
<em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">TimeoutError</span></span><a class="headerlink" href="#concurrent.futures.TimeoutError" title="Permalink to this definition">¶</a></dt>
<dd><p>Raised when a future operation exceeds the given timeout.</p>
</dd></dl>
<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.BrokenExecutor">
<em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">BrokenExecutor</span></span><a class="headerlink" href="#concurrent.futures.BrokenExecutor" title="Permalink to this definition">¶</a></dt>
<dd><p>Derived from <a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a>, this exception class is raised
when an executor is broken for some reason, and cannot be used
to submit or execute new tasks.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.InvalidStateError">
<em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.</span></span><span class="sig-name descname"><span class="pre">InvalidStateError</span></span><a class="headerlink" href="#concurrent.futures.InvalidStateError" title="Permalink to this definition">¶</a></dt>
<dd><p>Raised when an operation is performed on a future that is not allowed
in the current state.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.8.</span></p>
</div>
</dd></dl>
<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.thread.BrokenThreadPool">
<em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.thread.</span></span><span class="sig-name descname"><span class="pre">BrokenThreadPool</span></span><a class="headerlink" href="#concurrent.futures.thread.BrokenThreadPool" title="Permalink to this definition">¶</a></dt>
<dd><p>Derived from <a class="reference internal" href="#concurrent.futures.BrokenExecutor" title="concurrent.futures.BrokenExecutor"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BrokenExecutor</span></code></a>, this exception
class is raised when one of the workers of a <code class="xref py py-class docutils literal notranslate"><span class="pre">ThreadPoolExecutor</span></code>
has failed initializing.</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.7.</span></p>
</div>
</dd></dl>
<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.process.BrokenProcessPool">
<em class="property"><span class="pre">exception</span><span class="w"> </span></em><span class="sig-prename descclassname"><span class="pre">concurrent.futures.process.</span></span><span class="sig-name descname"><span class="pre">BrokenProcessPool</span></span><a class="headerlink" href="#concurrent.futures.process.BrokenProcessPool" title="Permalink to this definition">¶</a></dt>
<dd><p>Derived from <a class="reference internal" href="#concurrent.futures.BrokenExecutor" title="concurrent.futures.BrokenExecutor"><code class="xref py py-exc docutils literal notranslate"><span class="pre">BrokenExecutor</span></code></a> (formerly
<a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate"><span class="pre">RuntimeError</span></code></a>), this exception class is raised when one of the
workers of a <code class="xref py py-class docutils literal notranslate"><span class="pre">ProcessPoolExecutor</span></code> has terminated in a non-clean
fashion (for example, if it was killed from the outside).</p>
<div class="versionadded">
<p><span class="versionmodified added">New in version 3.3.</span></p>
</div>
</dd></dl>
</section>
</section>
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<h3><a href="../contents.html">Table of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate"><span class="pre">concurrent.futures</span></code> — Launching parallel tasks</a><ul>
<li><a class="reference internal" href="#executor-objects">Executor Objects</a></li>
<li><a class="reference internal" href="#threadpoolexecutor">ThreadPoolExecutor</a><ul>
<li><a class="reference internal" href="#threadpoolexecutor-example">ThreadPoolExecutor Example</a></li>
</ul>
</li>
<li><a class="reference internal" href="#processpoolexecutor">ProcessPoolExecutor</a><ul>
<li><a class="reference internal" href="#processpoolexecutor-example">ProcessPoolExecutor Example</a></li>
</ul>
</li>
<li><a class="reference internal" href="#future-objects">Future Objects</a></li>
<li><a class="reference internal" href="#module-functions">Module Functions</a></li>
<li><a class="reference internal" href="#exception-classes">Exception classes</a></li>
</ul>
</li>
</ul>
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