<!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="Content-Type" content="text/html; charset=utf-8" />
<title>16.4. concurrent.futures — Launching parallel tasks — Python v3.2.2 documentation</title>
<link rel="stylesheet" href="../_static/default.css" type="text/css" />
<link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
<script type="text/javascript">
var DOCUMENTATION_OPTIONS = {
URL_ROOT: '../',
VERSION: '3.2.2',
COLLAPSE_INDEX: false,
FILE_SUFFIX: '.html',
HAS_SOURCE: true
};
</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 v3.2.2 documentation"
href="../_static/opensearch.xml"/>
<link rel="author" title="About these documents" href="../about.html" />
<link rel="copyright" title="Copyright" href="../copyright.html" />
<link rel="top" title="Python v3.2.2 documentation" href="../index.html" />
<link rel="up" title="16. Optional Operating System Services" href="someos.html" />
<link rel="next" title="16.5. mmap — Memory-mapped file support" href="mmap.html" />
<link rel="prev" title="16.3. multiprocessing — Process-based parallelism" href="multiprocessing.html" />
<link rel="shortcut icon" type="image/png" href="../_static/py.png" />
</head>
<body>
<div class="related">
<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="mmap.html" title="16.5. mmap — Memory-mapped file support"
accesskey="N">next</a> |</li>
<li class="right" >
<a href="multiprocessing.html" title="16.3. multiprocessing — Process-based parallelism"
accesskey="P">previous</a> |</li>
<li><img src="../_static/py.png" alt=""
style="vertical-align: middle; margin-top: -1px"/></li>
<li><a href="../index.html">Python v3.2.2 documentation</a> »</li>
<li><a href="index.html" >The Python Standard Library</a> »</li>
<li><a href="someos.html" accesskey="U">16. Optional Operating System Services</a> »</li>
</ul>
</div>
<div class="document">
<div class="documentwrapper">
<div class="bodywrapper">
<div class="body">
<div class="section" id="module-concurrent.futures">
<span id="concurrent-futures-launching-parallel-tasks"></span><h1>16.4. <a class="reference internal" href="#module-concurrent.futures" title="concurrent.futures: Execute computations concurrently using threads or processes."><tt class="xref py py-mod docutils literal"><span class="pre">concurrent.futures</span></tt></a> — Launching parallel tasks<a class="headerlink" href="#module-concurrent.futures" title="Permalink to this headline">¶</a></h1>
<p class="versionadded">
<span class="versionmodified">New in version 3.2.</span></p>
<p><strong>Source code:</strong> <a class="reference external" href="http://hg.python.org/cpython/file/3.2/Lib/concurrent/futures/thread.py">Lib/concurrent/futures/thread.py</a>
and <a class="reference external" href="http://hg.python.org/cpython/file/3.2/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."><tt class="xref py py-mod docutils literal"><span class="pre">concurrent.futures</span></tt></a> module provides a high-level interface for
asynchronously executing callables.</p>
<p>The asynchronous execution can be be performed with threads, using
<a class="reference internal" href="#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><tt class="xref py py-class docutils literal"><span class="pre">ThreadPoolExecutor</span></tt></a>, or separate processes, using
<a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><tt class="xref py py-class docutils literal"><span class="pre">ProcessPoolExecutor</span></tt></a>. Both implement the same interface, which is
defined by the abstract <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> class.</p>
<div class="section" id="executor-objects">
<h2>16.4.1. Executor Objects<a class="headerlink" href="#executor-objects" title="Permalink to this headline">¶</a></h2>
<dl class="class">
<dt id="concurrent.futures.Executor">
<em class="property">class </em><tt class="descclassname">concurrent.futures.</tt><tt class="descname">Executor</tt><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="method">
<dt id="concurrent.futures.Executor.submit">
<tt class="descname">submit</tt><big>(</big><em>fn</em>, <em>*args</em>, <em>**kwargs</em><big>)</big><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 <tt class="docutils literal"><span class="pre">fn(*args</span> <span class="pre">**kwargs)</span></tt>
and returns a <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> object representing the execution of the
callable.</p>
<div class="highlight-python3"><div class="highlight"><pre><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="method">
<dt id="concurrent.futures.Executor.map">
<tt class="descname">map</tt><big>(</big><em>func</em>, <em>*iterables</em>, <em>timeout=None</em><big>)</big><a class="headerlink" href="#concurrent.futures.Executor.map" title="Permalink to this definition">¶</a></dt>
<dd><p>Equivalent to <tt class="docutils literal"><span class="pre">map(func,</span> <span class="pre">*iterables)</span></tt> except <em>func</em> is executed
asynchronously and several calls to <em>func</em> may be made concurrently. The
returned iterator raises a <tt class="xref py py-exc docutils literal"><span class="pre">TimeoutError</span></tt> if <tt class="xref py py-meth docutils literal"><span class="pre">__next__()</span></tt> 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"><tt class="xref py py-meth docutils literal"><span class="pre">Executor.map()</span></tt></a>. <em>timeout</em> can be an int or a
float. If <em>timeout</em> is not specified or <tt class="xref docutils literal"><span class="pre">None</span></tt>, there is no limit to
the wait time. If a call raises an exception, then that exception will
be raised when its value is retrieved from the iterator.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Executor.shutdown">
<tt class="descname">shutdown</tt><big>(</big><em>wait=True</em><big>)</big><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"><tt class="xref py py-meth docutils literal"><span class="pre">Executor.submit()</span></tt></a> and <a class="reference internal" href="#concurrent.futures.Executor.map" title="concurrent.futures.Executor.map"><tt class="xref py py-meth docutils literal"><span class="pre">Executor.map()</span></tt></a> made after shutdown will
raise <a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><tt class="xref py py-exc docutils literal"><span class="pre">RuntimeError</span></tt></a>.</p>
<p>If <em>wait</em> is <tt class="xref docutils literal"><span class="pre">True</span></tt> 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 <tt class="xref docutils literal"><span class="pre">False</span></tt> 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>You can avoid having to call this method explicitly if you use the
<a class="reference internal" href="../reference/compound_stmts.html#with"><tt class="xref std std-keyword docutils literal"><span class="pre">with</span></tt></a> statement, which will shutdown the <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a>
(waiting as if <a class="reference internal" href="#concurrent.futures.Executor.shutdown" title="concurrent.futures.Executor.shutdown"><tt class="xref py py-meth docutils literal"><span class="pre">Executor.shutdown()</span></tt></a> were called with <em>wait</em> set to
<tt class="xref docutils literal"><span class="pre">True</span></tt>):</p>
<div class="highlight-python3"><div class="highlight"><pre><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="s">'src1.txt'</span><span class="p">,</span> <span class="s">'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="s">'src2.txt'</span><span class="p">,</span> <span class="s">'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="s">'src3.txt'</span><span class="p">,</span> <span class="s">'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="s">'src3.txt'</span><span class="p">,</span> <span class="s">'dest4.txt'</span><span class="p">)</span>
</pre></div>
</div>
</dd></dl>
</div></blockquote>
</dd></dl>
</div>
<div class="section" id="threadpoolexecutor">
<h2>16.4.2. 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"><tt class="xref py py-class docutils literal"><span class="pre">ThreadPoolExecutor</span></tt></a> is a <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></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"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> waits on
the results of another <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a>. For example:</p>
<div class="highlight-python3"><div class="highlight"><pre><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="c"># 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="c"># 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"><div class="highlight"><pre><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="c"># This will never complete because there is only one worker thread and</span>
<span class="c"># 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="class">
<dt id="concurrent.futures.ThreadPoolExecutor">
<em class="property">class </em><tt class="descclassname">concurrent.futures.</tt><tt class="descname">ThreadPoolExecutor</tt><big>(</big><em>max_workers</em><big>)</big><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"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> subclass that uses a pool of at most <em>max_workers</em>
threads to execute calls asynchronously.</p>
</dd></dl>
<div class="section" id="threadpoolexecutor-example">
<span id="id1"></span><h3>16.4.2.1. ThreadPoolExecutor Example<a class="headerlink" href="#threadpoolexecutor-example" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python3"><div class="highlight"><pre><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="s">'http://www.foxnews.com/'</span><span class="p">,</span>
<span class="s">'http://www.cnn.com/'</span><span class="p">,</span>
<span class="s">'http://europe.wsj.com/'</span><span class="p">,</span>
<span class="s">'http://www.bbc.co.uk/'</span><span class="p">,</span>
<span class="s">'http://some-made-up-domain.com/'</span><span class="p">]</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">return</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="o">.</span><span class="n">read</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">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="n">future_to_url</span> <span class="o">=</span> <span class="nb">dict</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="p">)</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">if</span> <span class="n">future</span><span class="o">.</span><span class="n">exception</span><span class="p">()</span> <span class="ow">is</span> <span class="ow">not</span> <span class="k">None</span><span class="p">:</span>
<span class="nb">print</span><span class="p">(</span><span class="s">'%r generated an exception: %s'</span> <span class="o">%</span> <span class="p">(</span><span class="n">url</span><span class="p">,</span>
<span class="n">future</span><span class="o">.</span><span class="n">exception</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="s">'%r page is %d 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">future</span><span class="o">.</span><span class="n">result</span><span class="p">())))</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="processpoolexecutor">
<h2>16.4.3. 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"><tt class="xref py py-class docutils literal"><span class="pre">ProcessPoolExecutor</span></tt></a> class is an <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> subclass that
uses a pool of processes to execute calls asynchronously.
<a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><tt class="xref py py-class docutils literal"><span class="pre">ProcessPoolExecutor</span></tt></a> uses the <a class="reference internal" href="multiprocessing.html#module-multiprocessing" title="multiprocessing: Process-based parallelism."><tt class="xref py py-mod docutils literal"><span class="pre">multiprocessing</span></tt></a> module, which
allows it to side-step the <a class="reference internal" href="../glossary.html#term-global-interpreter-lock"><em class="xref std std-term">Global Interpreter Lock</em></a> but also means that
only picklable objects can be executed and returned.</p>
<p>Calling <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> or <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> methods from a callable submitted
to a <a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><tt class="xref py py-class docutils literal"><span class="pre">ProcessPoolExecutor</span></tt></a> will result in deadlock.</p>
<dl class="class">
<dt id="concurrent.futures.ProcessPoolExecutor">
<em class="property">class </em><tt class="descclassname">concurrent.futures.</tt><tt class="descname">ProcessPoolExecutor</tt><big>(</big><em>max_workers=None</em><big>)</big><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"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> subclass that executes calls asynchronously using a pool
of at most <em>max_workers</em> processes. If <em>max_workers</em> is <tt class="xref docutils literal"><span class="pre">None</span></tt> or not
given, it will default to the number of processors on the machine.</p>
</dd></dl>
<div class="section" id="processpoolexecutor-example">
<span id="id2"></span><h3>16.4.3.1. ProcessPoolExecutor Example<a class="headerlink" href="#processpoolexecutor-example" title="Permalink to this headline">¶</a></h3>
<div class="highlight-python3"><div class="highlight"><pre><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="o">==</span> <span class="mi">0</span><span class="p">:</span>
<span class="k">return</span> <span class="k">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="k">False</span>
<span class="k">return</span> <span class="k">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="s">'%d is prime: %s'</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="n">__name__</span> <span class="o">==</span> <span class="s">'__main__'</span><span class="p">:</span>
<span class="n">main</span><span class="p">()</span>
</pre></div>
</div>
</div>
</div>
<div class="section" id="future-objects">
<h2>16.4.4. 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"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> class encapsulates the asynchronous execution of a callable.
<a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> instances are created by <a class="reference internal" href="#concurrent.futures.Executor.submit" title="concurrent.futures.Executor.submit"><tt class="xref py py-meth docutils literal"><span class="pre">Executor.submit()</span></tt></a>.</p>
<dl class="class">
<dt id="concurrent.futures.Future">
<em class="property">class </em><tt class="descclassname">concurrent.futures.</tt><tt class="descname">Future</tt><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"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a>
instances are created by <a class="reference internal" href="#concurrent.futures.Executor.submit" title="concurrent.futures.Executor.submit"><tt class="xref py py-meth docutils literal"><span class="pre">Executor.submit()</span></tt></a> and should not be created
directly except for testing.</p>
<blockquote>
<div><dl class="method">
<dt id="concurrent.futures.Future.cancel">
<tt class="descname">cancel</tt><big>(</big><big>)</big><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 and
cannot be cancelled then the method will return <tt class="xref docutils literal"><span class="pre">False</span></tt>, otherwise the
call will be cancelled and the method will return <tt class="xref docutils literal"><span class="pre">True</span></tt>.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Future.cancelled">
<tt class="descname">cancelled</tt><big>(</big><big>)</big><a class="headerlink" href="#concurrent.futures.Future.cancelled" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <tt class="xref docutils literal"><span class="pre">True</span></tt> if the call was successfully cancelled.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Future.running">
<tt class="descname">running</tt><big>(</big><big>)</big><a class="headerlink" href="#concurrent.futures.Future.running" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <tt class="xref docutils literal"><span class="pre">True</span></tt> if the call is currently being executed and cannot be
cancelled.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Future.done">
<tt class="descname">done</tt><big>(</big><big>)</big><a class="headerlink" href="#concurrent.futures.Future.done" title="Permalink to this definition">¶</a></dt>
<dd><p>Return <tt class="xref docutils literal"><span class="pre">True</span></tt> if the call was successfully cancelled or finished
running.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Future.result">
<tt class="descname">result</tt><big>(</big><em>timeout=None</em><big>)</big><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 <tt class="xref py py-exc docutils literal"><span class="pre">TimeoutError</span></tt> will be
raised. <em>timeout</em> can be an int or float. If <em>timeout</em> is not specified
or <tt class="xref docutils literal"><span class="pre">None</span></tt>, there is no limit to the wait time.</p>
<p>If the future is cancelled before completing then <tt class="xref py py-exc docutils literal"><span class="pre">CancelledError</span></tt>
will be raised.</p>
<p>If the call raised, this method will raise the same exception.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Future.exception">
<tt class="descname">exception</tt><big>(</big><em>timeout=None</em><big>)</big><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 <tt class="xref py py-exc docutils literal"><span class="pre">TimeoutError</span></tt>
will be raised. <em>timeout</em> can be an int or float. If <em>timeout</em> is not
specified or <tt class="xref docutils literal"><span class="pre">None</span></tt>, there is no limit to the wait time.</p>
<p>If the future is cancelled before completing then <tt class="xref py py-exc docutils literal"><span class="pre">CancelledError</span></tt>
will be raised.</p>
<p>If the call completed without raising, <tt class="xref docutils literal"><span class="pre">None</span></tt> is returned.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Future.add_done_callback">
<tt class="descname">add_done_callback</tt><big>(</big><em>fn</em><big>)</big><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 a <a class="reference internal" href="exceptions.html#Exception" title="Exception"><tt class="xref py py-exc docutils literal"><span class="pre">Exception</span></tt></a> subclass, it will be logged and
ignored. If the callable raises a <a class="reference internal" href="exceptions.html#BaseException" title="BaseException"><tt class="xref py py-exc docutils literal"><span class="pre">BaseException</span></tt></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"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> methods are meant for use in unit tests and
<a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> implementations.</p>
<blockquote>
<div><dl class="method">
<dt id="concurrent.futures.Future.set_running_or_notify_cancel">
<tt class="descname">set_running_or_notify_cancel</tt><big>(</big><big>)</big><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"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> implementations
before executing the work associated with the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> and by unit
tests.</p>
<p>If the method returns <tt class="xref docutils literal"><span class="pre">False</span></tt> then the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> was cancelled,
i.e. <a class="reference internal" href="#concurrent.futures.Future.cancel" title="concurrent.futures.Future.cancel"><tt class="xref py py-meth docutils literal"><span class="pre">Future.cancel()</span></tt></a> was called and returned <cite>True</cite>. Any threads
waiting on the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> completing (i.e. through
<a class="reference internal" href="#concurrent.futures.as_completed" title="concurrent.futures.as_completed"><tt class="xref py py-func docutils literal"><span class="pre">as_completed()</span></tt></a> or <a class="reference internal" href="#concurrent.futures.wait" title="concurrent.futures.wait"><tt class="xref py py-func docutils literal"><span class="pre">wait()</span></tt></a>) will be woken up.</p>
<p>If the method returns <tt class="xref docutils literal"><span class="pre">True</span></tt> then the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></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"><tt class="xref py py-meth docutils literal"><span class="pre">Future.running()</span></tt></a> will return <cite>True</cite>.</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"><tt class="xref py py-meth docutils literal"><span class="pre">Future.set_result()</span></tt></a> or <a class="reference internal" href="#concurrent.futures.Future.set_exception" title="concurrent.futures.Future.set_exception"><tt class="xref py py-meth docutils literal"><span class="pre">Future.set_exception()</span></tt></a> have been
called.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Future.set_result">
<tt class="descname">set_result</tt><big>(</big><em>result</em><big>)</big><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"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></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"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> implementations and
unit tests.</p>
</dd></dl>
<dl class="method">
<dt id="concurrent.futures.Future.set_exception">
<tt class="descname">set_exception</tt><big>(</big><em>exception</em><big>)</big><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"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> to the
<a class="reference internal" href="exceptions.html#Exception" title="Exception"><tt class="xref py py-class docutils literal"><span class="pre">Exception</span></tt></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"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> implementations and
unit tests.</p>
</dd></dl>
</div></blockquote>
</dd></dl>
</div>
<div class="section" id="module-functions">
<h2>16.4.5. Module Functions<a class="headerlink" href="#module-functions" title="Permalink to this headline">¶</a></h2>
<dl class="function">
<dt id="concurrent.futures.wait">
<tt class="descclassname">concurrent.futures.</tt><tt class="descname">wait</tt><big>(</big><em>fs</em>, <em>timeout=None</em>, <em>return_when=ALL_COMPLETED</em><big>)</big><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"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> instances (possibly created by different
<a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> instances) given by <em>fs</em> to complete. Returns a named
2-tuple of sets. The first set, named <tt class="docutils literal"><span class="pre">done</span></tt>, contains the futures that
completed (finished or were cancelled) before the wait completed. The second
set, named <tt class="docutils literal"><span class="pre">not_done</span></tt>, contains uncompleted 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 <tt class="xref docutils literal"><span class="pre">None</span></tt>, 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 border="1" class="docutils">
<colgroup>
<col width="42%" />
<col width="58%" />
</colgroup>
<thead valign="bottom">
<tr><th class="head">Constant</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody valign="top">
<tr><td><tt class="xref py py-const docutils literal"><span class="pre">FIRST_COMPLETED</span></tt></td>
<td>The function will return when any
future finishes or is cancelled.</td>
</tr>
<tr><td><tt class="xref py py-const docutils literal"><span class="pre">FIRST_EXCEPTION</span></tt></td>
<td>The function will return when any
future finishes by raising an
exception. If no future raises an
exception then it is equivalent to
<tt class="xref py py-const docutils literal"><span class="pre">ALL_COMPLETED</span></tt>.</td>
</tr>
<tr><td><tt class="xref py py-const docutils literal"><span class="pre">ALL_COMPLETED</span></tt></td>
<td>The function will return when all
futures finish or are cancelled.</td>
</tr>
</tbody>
</table>
</dd></dl>
<dl class="function">
<dt id="concurrent.futures.as_completed">
<tt class="descclassname">concurrent.futures.</tt><tt class="descname">as_completed</tt><big>(</big><em>fs</em>, <em>timeout=None</em><big>)</big><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"><tt class="xref py py-class docutils literal"><span class="pre">Future</span></tt></a> instances (possibly created by
different <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><tt class="xref py py-class docutils literal"><span class="pre">Executor</span></tt></a> instances) given by <em>fs</em> that yields futures as
they complete (finished or were cancelled). Any futures that completed
before <a class="reference internal" href="#concurrent.futures.as_completed" title="concurrent.futures.as_completed"><tt class="xref py py-func docutils literal"><span class="pre">as_completed()</span></tt></a> is called will be yielded first. The returned
iterator raises a <tt class="xref py py-exc docutils literal"><span class="pre">TimeoutError</span></tt> if <tt class="xref py py-meth docutils literal"><span class="pre">__next__()</span></tt> 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"><tt class="xref py py-func docutils literal"><span class="pre">as_completed()</span></tt></a>. <em>timeout</em> can be an int or float. If <em>timeout</em> is not
specified or <tt class="xref docutils literal"><span class="pre">None</span></tt>, there is no limit to the wait time.</p>
</dd></dl>
<div class="admonition-see-also admonition seealso">
<p class="first admonition-title">See also</p>
<dl class="last docutils">
<dt><span class="target" id="index-0"></span><a class="pep reference external" href="http://www.python.org/dev/peps/pep-3148"><strong>PEP 3148</strong></a> – futures - execute computations asynchronously</dt>
<dd>The proposal which described this feature for inclusion in the Python
standard library.</dd>
</dl>
</div>
</div>
</div>
</div>
</div>
</div>
<div class="sphinxsidebar">
<div class="sphinxsidebarwrapper">
<h3><a href="../contents.html">Table Of Contents</a></h3>
<ul>
<li><a class="reference internal" href="#">16.4. <tt class="docutils literal"><span class="pre">concurrent.futures</span></tt> — Launching parallel tasks</a><ul>
<li><a class="reference internal" href="#executor-objects">16.4.1. Executor Objects</a></li>
<li><a class="reference internal" href="#threadpoolexecutor">16.4.2. ThreadPoolExecutor</a><ul>
<li><a class="reference internal" href="#threadpoolexecutor-example">16.4.2.1. ThreadPoolExecutor Example</a></li>
</ul>
</li>
<li><a class="reference internal" href="#processpoolexecutor">16.4.3. ProcessPoolExecutor</a><ul>
<li><a class="reference internal" href="#processpoolexecutor-example">16.4.3.1. ProcessPoolExecutor Example</a></li>
</ul>
</li>
<li><a class="reference internal" href="#future-objects">16.4.4. Future Objects</a></li>
<li><a class="reference internal" href="#module-functions">16.4.5. Module Functions</a></li>
</ul>
</li>
</ul>
<h4>Previous topic</h4>
<p class="topless"><a href="multiprocessing.html"
title="previous chapter">16.3. <tt class="docutils literal"><span class="pre">multiprocessing</span></tt> — Process-based parallelism</a></p>
<h4>Next topic</h4>
<p class="topless"><a href="mmap.html"
title="next chapter">16.5. <tt class="docutils literal"><span class="pre">mmap</span></tt> — Memory-mapped file support</a></p>
<h3>This Page</h3>
<ul class="this-page-menu">
<li><a href="../bugs.html">Report a Bug</a></li>
<li><a href="../_sources/library/concurrent.futures.txt"
rel="nofollow">Show Source</a></li>
</ul>
<div id="searchbox" style="display: none">
<h3>Quick search</h3>
<form class="search" action="../search.html" method="get">
<input type="text" name="q" size="18" />
<input type="submit" value="Go" />
<input type="hidden" name="check_keywords" value="yes" />
<input type="hidden" name="area" value="default" />
</form>
<p class="searchtip" style="font-size: 90%">
Enter search terms or a module, class or function name.
</p>
</div>
<script type="text/javascript">$('#searchbox').show(0);</script>
</div>
</div>
<div class="clearer"></div>
</div>
<div class="related">
<h3>Navigation</h3>
<ul>
<li class="right" style="margin-right: 10px">
<a href="../genindex.html" title="General Index"
>index</a></li>
<li class="right" >
<a href="../py-modindex.html" title="Python Module Index"
>modules</a> |</li>
<li class="right" >
<a href="mmap.html" title="16.5. mmap — Memory-mapped file support"
>next</a> |</li>
<li class="right" >
<a href="multiprocessing.html" title="16.3. multiprocessing — Process-based parallelism"
>previous</a> |</li>
<li><img src="../_static/py.png" alt=""
style="vertical-align: middle; margin-top: -1px"/></li>
<li><a href="../index.html">Python v3.2.2 documentation</a> »</li>
<li><a href="index.html" >The Python Standard Library</a> »</li>
<li><a href="someos.html" >16. Optional Operating System Services</a> »</li>
</ul>
</div>
<div class="footer">
© <a href="../copyright.html">Copyright</a> 1990-2011, Python Software Foundation.
<br />
The Python Software Foundation is a non-profit corporation.
<a href="http://www.python.org/psf/donations/">Please donate.</a>
<br />
Last updated on Sep 04, 2011.
<a href="../bugs.html">Found a bug</a>?
<br />
Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.0.7.
</div>
</body>
</html>
