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<title>Process Pools</title>
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<div class="document" id="process-pools">
<h1 class="title">Process Pools</h1>
<p>The <tt class="docutils literal"><span class="pre">processing.pool</span></tt> module has one public class:</p>
<blockquote>
<dl class="docutils">
<dt><strong>class</strong> <tt class="docutils literal"><span class="pre">Pool(processes=None,</span> <span class="pre">initializer=None,</span> <span class="pre">initargs=())</span></tt></dt>
<dd><p class="first">A class representing a pool of worker processes.</p>
<p>Tasks can be offloaded to the pool and the results dealt with
when they become available.</p>
<p>Note that tasks can only be submitted (or retrieved) by the
process which created the pool object.</p>
<p class="last"><tt class="docutils literal"><span class="pre">processes</span></tt> is the number of worker processes to use. If
<tt class="docutils literal"><span class="pre">processes</span></tt> is <tt class="docutils literal"><span class="pre">None</span></tt> then the number returned by <tt class="docutils literal"><span class="pre">cpuCount()</span></tt>
is used. If <tt class="docutils literal"><span class="pre">initializer</span></tt> is not <tt class="docutils literal"><span class="pre">None</span></tt> then each worker
process will call <tt class="docutils literal"><span class="pre">initializer(*initargs)</span></tt> when it starts.</p>
</dd>
</dl>
</blockquote>
<div class="section">
<h1><a id="pool-objects" name="pool-objects">Pool objects</a></h1>
<p><tt class="docutils literal"><span class="pre">Pool</span></tt> has the following public methods:</p>
<blockquote>
<dl class="docutils">
<dt><tt class="docutils literal"><span class="pre">__init__(processes=None)</span></tt></dt>
<dd>The constructor creates and starts <tt class="docutils literal"><span class="pre">processes</span></tt> worker
processes. If <tt class="docutils literal"><span class="pre">processes</span></tt> is <tt class="docutils literal"><span class="pre">None</span></tt> then <tt class="docutils literal"><span class="pre">cpuCount()</span></tt> is used
to find a default or 1 if <tt class="docutils literal"><span class="pre">cpuCount()</span></tt> raises <tt class="docutils literal"><span class="pre">NotImplemented</span></tt>.</dd>
<dt><tt class="docutils literal"><span class="pre">apply(func,</span> <span class="pre">args=(),</span> <span class="pre">kwds={})</span></tt></dt>
<dd>Equivalent of the <tt class="docutils literal"><span class="pre">apply()</span></tt> builtin function. It blocks till
the result is ready.</dd>
<dt><tt class="docutils literal"><span class="pre">applyAsync(func,</span> <span class="pre">args=(),</span> <span class="pre">kwds={},</span> <span class="pre">callback=None)</span></tt></dt>
<dd><p class="first">A variant of the <tt class="docutils literal"><span class="pre">apply()</span></tt> method which returns a
result object --- see <a class="reference" href="#asynchronous-result-objects">Asynchronous result objects</a>.</p>
<p class="last">If <tt class="docutils literal"><span class="pre">callback</span></tt> is specified then it should be a callable which
accepts a single argument. When the result becomes ready
<tt class="docutils literal"><span class="pre">callback</span></tt> is applied to it (unless the call failed).
<tt class="docutils literal"><span class="pre">callback</span></tt> should complete immediately since otherwise the
thread which handles the results will get blocked.</p>
</dd>
<dt><tt class="docutils literal"><span class="pre">map(func,</span> <span class="pre">iterable,</span> <span class="pre">chunksize=None)</span></tt></dt>
<dd><p class="first">A parallel equivalent of the <tt class="docutils literal"><span class="pre">map()</span></tt> builtin function. It
blocks till the result is ready.</p>
<p class="last">This method chops the iterable into a number of chunks which
it submits to the process pool as separate tasks. The
(approximate) size of these chunks can be specified by setting
<tt class="docutils literal"><span class="pre">chunksize</span></tt> to a positive integer.</p>
</dd>
<dt><tt class="docutils literal"><span class="pre">mapAsync(func,</span> <span class="pre">iterable,</span> <span class="pre">chunksize=None,</span> <span class="pre">callback=None)</span></tt></dt>
<dd><p class="first">A variant of the <tt class="docutils literal"><span class="pre">map()</span></tt> method which returns a result object
--- see <a class="reference" href="#asynchronous-result-objects">Asynchronous result objects</a>.</p>
<p class="last">If <tt class="docutils literal"><span class="pre">callback</span></tt> is specified then it should be a callable which
accepts a single argument. When the result becomes ready
<tt class="docutils literal"><span class="pre">callback</span></tt> is applied to it (unless the call failed).
<tt class="docutils literal"><span class="pre">callback</span></tt> should complete immediately since otherwise the
thread which handles the results will get blocked.</p>
</dd>
<dt><tt class="docutils literal"><span class="pre">imap(func,</span> <span class="pre">iterable,</span> <span class="pre">chunksize=1)</span></tt></dt>
<dd><p class="first">An equivalent of <tt class="docutils literal"><span class="pre">itertools.imap()</span></tt>.</p>
<p>The <tt class="docutils literal"><span class="pre">chunksize</span></tt> argument is the same as the one used by the
<tt class="docutils literal"><span class="pre">map()</span></tt> method. For very long iterables using a large value
for <tt class="docutils literal"><span class="pre">chunksize</span></tt> can make make the job complete <strong>much</strong> faster
than using the default value of <tt class="docutils literal"><span class="pre">1</span></tt>.</p>
<p class="last">Also if <tt class="docutils literal"><span class="pre">chunksize</span></tt> is <tt class="docutils literal"><span class="pre">1</span></tt> then the <tt class="docutils literal"><span class="pre">next()</span></tt> method of the
iterator returned by the <tt class="docutils literal"><span class="pre">imap()</span></tt> method has an optional
<tt class="docutils literal"><span class="pre">timeout</span></tt> parameter: <tt class="docutils literal"><span class="pre">next(timeout)</span></tt> will raise
<tt class="docutils literal"><span class="pre">processing.TimeoutError</span></tt> if the result cannot be returned
within <tt class="docutils literal"><span class="pre">timeout</span></tt> seconds.</p>
</dd>
<dt><tt class="docutils literal"><span class="pre">imapUnordered(func,</span> <span class="pre">iterable,</span> <span class="pre">chunksize=1)</span></tt></dt>
<dd>The same as <tt class="docutils literal"><span class="pre">imap()</span></tt> except that the ordering of the results
from the returned iterator should be considered arbitrary.
(Only when there is only one worker process is the order
guaranteed to be "correct".)</dd>
<dt><tt class="docutils literal"><span class="pre">close()</span></tt></dt>
<dd>Prevents any more tasks from being submitted to the pool.
Once all the tasks have been completed the worker processes
will exit.</dd>
<dt><tt class="docutils literal"><span class="pre">terminate()</span></tt></dt>
<dd>Stops the worker processes immediately without completing
outstanding work. When the pool object is garbage collected
<tt class="docutils literal"><span class="pre">terminate()</span></tt> will be called immediately.</dd>
<dt><tt class="docutils literal"><span class="pre">join()</span></tt></dt>
<dd>Wait for the worker processes to exit. One must call
<tt class="docutils literal"><span class="pre">close()</span></tt> or <tt class="docutils literal"><span class="pre">terminate()</span></tt> before using <tt class="docutils literal"><span class="pre">join()</span></tt>.</dd>
</dl>
</blockquote>
</div>
<div class="section">
<h1><a id="asynchronous-result-objects" name="asynchronous-result-objects">Asynchronous result objects</a></h1>
<p>The result objects returns by <tt class="docutils literal"><span class="pre">applyAsync()</span></tt> and <tt class="docutils literal"><span class="pre">mapAsync()</span></tt> have
the following public methods:</p>
<blockquote>
<dl class="docutils">
<dt><tt class="docutils literal"><span class="pre">get(timeout=None)</span></tt></dt>
<dd>Returns the result when it arrives. If <tt class="docutils literal"><span class="pre">timeout</span></tt> is not
<tt class="docutils literal"><span class="pre">None</span></tt> and the result does not arrive within <tt class="docutils literal"><span class="pre">timeout</span></tt> seconds
then <tt class="docutils literal"><span class="pre">processing.TimeoutError</span></tt> is raised. If the remote call
raised an exception then that exception will be reraised by <tt class="docutils literal"><span class="pre">get()</span></tt>.</dd>
<dt><tt class="docutils literal"><span class="pre">wait(timeout=None)</span></tt></dt>
<dd>Waits until the result is available or until <tt class="docutils literal"><span class="pre">timeout</span></tt> seconds
pass.</dd>
<dt><tt class="docutils literal"><span class="pre">ready()</span></tt></dt>
<dd>Returns whether the call has completed.</dd>
<dt><tt class="docutils literal"><span class="pre">successful()</span></tt></dt>
<dd>Returns whether the call completed without raising an
exception. Will raise <tt class="docutils literal"><span class="pre">AssertionError</span></tt> if the result is not
ready.</dd>
</dl>
</blockquote>
</div>
<div class="section">
<h1><a id="examples" name="examples">Examples</a></h1>
<p>The following example demonstrates the use of a pool:</p>
<pre class="literal-block">
from processing import Pool
def f(x):
return x*x
if __name__ == '__main__':
pool = Pool(processes=4) # start 4 worker processes
result = pool.applyAsync(f, (10,)) # evaluate "f(10)" asynchronously
print result.get(timeout=1) # prints "100" unless your computer is *very* slow
print pool.map(f, range(10)) # prints "[0, 1, 4,..., 81]"
it = pool.imap(f, range(10))
print it.next() # prints "0"
print it.next() # prints "1"
print it.next(timeout=1) # prints "4" unless your computer is *very* slow
import time
result = pool.applyAsync(time.sleep, (10,))
print result.get(timeout=1) # raises `TimeoutError`
</pre>
<p>See also <a class="reference" href="../examples/ex_pool.py">ex_pool.py</a>.</p>
</div>
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