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<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate">concurrent.futures</code> — Launching parallel tasks</a><ul>
<li><a class="reference internal" href="#executor-objects">Executor Objects</a></li>
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 <section id="module-concurrent.futures">
<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">concurrent.futures</code></a> — Launching parallel tasks<a class="headerlink" href="#module-concurrent.futures" title="Permalink to this headline">¶</a></h1>
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New in version 3.2.
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Source code: <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>
<hr class="docutils" />
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">concurrent.futures</code></a> module provides a high-level interface for
asynchronously executing callables.
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">ThreadPoolExecutor</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">ProcessPoolExecutor</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">Executor</code></a> class.
<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">
class concurrent.futures.Executor<a class="headerlink" href="#concurrent.futures.Executor" title="Permalink to this definition">¶</a></dt>
<dd>An abstract class that provides methods to execute calls asynchronously. It
should not be used directly, but through its concrete subclasses.
<blockquote>
<div><dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Executor.submit">
submit(fn, /, *args, **kwargs)<a class="headerlink" href="#concurrent.futures.Executor.submit" title="Permalink to this definition">¶</a></dt>
<dd>Schedules the callable, fn, to be executed as <code class="docutils literal notranslate">fn(*args, **kwargs)</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">Future</code></a> object representing the execution of the
callable.
<div class="highlight-python3 notranslate"><div class="highlight"><pre>with ThreadPoolExecutor(max_workers=1) as executor:
 future = executor.submit(pow, 323, 1235)
 print(future.result())
</pre></div>
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Executor.map">
map(func, *iterables, timeout=None, chunksize=1)<a class="headerlink" href="#concurrent.futures.Executor.map" title="Permalink to this definition">¶</a></dt>
<dd>Similar to <a class="reference internal" href="functions.html#map" title="map"><code class="xref py py-func docutils literal notranslate">map(func, *iterables)</code></a> except:
<ul class="simple">
<li>the iterables are collected immediately rather than lazily;</li>
<li>func is executed asynchronously and several calls to
func may be made concurrently.</li>
</ul>
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">concurrent.futures.TimeoutError</code></a>
if <a class="reference internal" href="stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate">__next__()</code></a> is called and the result isn’t available
after timeout 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">Executor.map()</code></a>.
timeout can be an int or a float. If timeout is not specified or
<code class="docutils literal notranslate">None</code>, there is no limit to the wait time.
If a func call raises an exception, then that exception will be
raised when its value is retrieved from the iterator.
When using <a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate">ProcessPoolExecutor</code></a>, this method chops iterables
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 chunksize to a positive integer. For very long iterables,
using a large value for chunksize 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">ThreadPoolExecutor</code></a>, chunksize has no effect.
<div class="versionchanged">
Changed in version 3.5: Added the chunksize argument.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Executor.shutdown">
shutdown(wait=True, *, cancel_futures=False)<a class="headerlink" href="#concurrent.futures.Executor.shutdown" title="Permalink to this definition">¶</a></dt>
<dd>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">Executor.submit()</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">Executor.map()</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">RuntimeError</code></a>.
If wait is <code class="docutils literal notranslate">True</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 wait is <code class="docutils literal notranslate">False</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 wait, the entire Python program will not exit until all
pending futures are done executing.
If cancel_futures is <code class="docutils literal notranslate">True</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 cancel_futures.
If both cancel_futures and wait are <code class="docutils literal notranslate">True</code>, all futures that the
executor has started running will be completed prior to this method
returning. The remaining futures are cancelled.
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">with</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">Executor</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">Executor.shutdown()</code></a> were called with wait set to
<code class="docutils literal notranslate">True</code>):
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import shutil
with ThreadPoolExecutor(max_workers=4) as e:
 e.submit(shutil.copy, &#39;src1.txt&#39;, &#39;dest1.txt&#39;)
 e.submit(shutil.copy, &#39;src2.txt&#39;, &#39;dest2.txt&#39;)
 e.submit(shutil.copy, &#39;src3.txt&#39;, &#39;dest3.txt&#39;)
 e.submit(shutil.copy, &#39;src4.txt&#39;, &#39;dest4.txt&#39;)
</pre></div>
</div>
<div class="versionchanged">
Changed in version 3.9: Added cancel_futures.
</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>
<a class="reference internal" href="#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate">ThreadPoolExecutor</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">Executor</code></a> subclass that uses a pool of
threads to execute calls asynchronously.
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">Future</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">Future</code></a>. For example:
<div class="highlight-python3 notranslate"><div class="highlight"><pre>import time
def wait_on_b():
 time.sleep(5)
 print(b.result()) # b will never complete because it is waiting on a.
 return 5

def wait_on_a():
 time.sleep(5)
 print(a.result()) # a will never complete because it is waiting on b.
 return 6

executor = ThreadPoolExecutor(max_workers=1)
executor.submit(wait_on_future)
</pre></div>
</div>
<dl class="py class">
<dt class="sig sig-object py" id="concurrent.futures.ThreadPoolExecutor">
class concurrent.futures.ThreadPoolExecutor(max_workers=None, thread_name_prefix='', initializer=None, initargs=())<a class="headerlink" href="#concurrent.futures.ThreadPoolExecutor" title="Permalink to this definition">¶</a></dt>
<dd>An <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate">Executor</code></a> subclass that uses a pool of at most max_workers
threads to execute calls asynchronously.
All threads enqueued to <code class="docutils literal notranslate">ThreadPoolExecutor</code> will be joined before the
interpreter can exit. Note that the exit handler which does this is
executed before any exit handlers added using <code class="docutils literal notranslate">atexit</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">ThreadPoolExecutor</code> not be used for long-running tasks.
initializer is an optional callable that is called at the start of
each worker thread; initargs is a tuple of arguments passed to the
initializer. Should initializer 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">BrokenThreadPool</code></a>,
as well as any attempt to submit more jobs to the pool.
<div class="versionchanged">
Changed in version 3.5: If max_workers is <code class="docutils literal notranslate">None</code> or
not given, it will default to the number of processors on the machine,
multiplied by <code class="docutils literal notranslate">5</code>, assuming that <a class="reference internal" href="#concurrent.futures.ThreadPoolExecutor" title="concurrent.futures.ThreadPoolExecutor"><code class="xref py py-class docutils literal notranslate">ThreadPoolExecutor</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">ProcessPoolExecutor</code></a>.
</div>
<div class="versionadded">
New in version 3.6: The thread_name_prefix 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">threading.Thread</code></a> names for worker threads created by
the pool for easier debugging.
</div>
<div class="versionchanged">
Changed in version 3.7: Added the initializer and initargs arguments.
</div>
<div class="versionchanged">
Changed in version 3.8: Default value of max_workers is changed to <code class="docutils literal notranslate">min(32, os.cpu_count() + 4)</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.
ThreadPoolExecutor now reuses idle worker threads before starting
max_workers worker threads too.
</div>
</dd></dl>

<section id="threadpoolexecutor-example">
<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>import concurrent.futures
import urllib.request

URLS = [&#39;http://www.foxnews.com/&#39;,
 &#39;http://www.cnn.com/&#39;,
 &#39;http://europe.wsj.com/&#39;,
 &#39;http://www.bbc.co.uk/&#39;,
 &#39;http://nonexistant-subdomain.python.org/&#39;]

# Retrieve a single page and report the URL and contents
def load_url(url, timeout):
 with urllib.request.urlopen(url, timeout=timeout) as conn:
 return conn.read()

# We can use a with statement to ensure threads are cleaned up promptly
with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
 # Start the load operations and mark each future with its URL
 future_to_url = {executor.submit(load_url, url, 60): url for url in URLS}
 for future in concurrent.futures.as_completed(future_to_url):
 url = future_to_url[future]
 try:
 data = future.result()
 except Exception as exc:
 print(&#39;%r generated an exception: %s&#39; % (url, exc))
 else:
 print(&#39;%r page is %d bytes&#39; % (url, len(data)))
</pre></div>
</div>
</section>
</section>
<section id="processpoolexecutor">
<h2>ProcessPoolExecutor<a class="headerlink" href="#processpoolexecutor" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="#concurrent.futures.ProcessPoolExecutor" title="concurrent.futures.ProcessPoolExecutor"><code class="xref py py-class docutils literal notranslate">ProcessPoolExecutor</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">Executor</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">ProcessPoolExecutor</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">multiprocessing</code></a> module, which
allows it to side-step the <a class="reference internal" href="../glossary.html#term-global-interpreter-lock">Global Interpreter Lock</a> but also means that
only picklable objects can be executed and returned.
The <code class="docutils literal notranslate">__main__</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">ProcessPoolExecutor</code></a> will not work in the interactive interpreter.
Calling <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate">Executor</code></a> or <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate">Future</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">ProcessPoolExecutor</code></a> will result in deadlock.
<dl class="py class">
<dt class="sig sig-object py" id="concurrent.futures.ProcessPoolExecutor">
class concurrent.futures.ProcessPoolExecutor(max_workers=None, mp_context=None, initializer=None, initargs=())<a class="headerlink" href="#concurrent.futures.ProcessPoolExecutor" title="Permalink to this definition">¶</a></dt>
<dd>An <a class="reference internal" href="#concurrent.futures.Executor" title="concurrent.futures.Executor"><code class="xref py py-class docutils literal notranslate">Executor</code></a> subclass that executes calls asynchronously using a pool
of at most max_workers processes. If max_workers is <code class="docutils literal notranslate">None</code> or not
given, it will default to the number of processors on the machine.
If max_workers is less than or equal to <code class="docutils literal notranslate">0</code>, then a <a class="reference internal" href="exceptions.html#ValueError" title="ValueError"><code class="xref py py-exc docutils literal notranslate">ValueError</code></a>
will be raised.
On Windows, max_workers must be less than or equal to <code class="docutils literal notranslate">61</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">ValueError</code></a> will be raised. If max_workers is <code class="docutils literal notranslate">None</code>, then
the default chosen will be at most <code class="docutils literal notranslate">61</code>, even if more processors are
available.
mp_context can be a multiprocessing context or None. It will be used to
launch the workers. If mp_context is <code class="docutils literal notranslate">None</code> or not given, the default
multiprocessing context is used.
initializer is an optional callable that is called at the start of
each worker process; initargs is a tuple of arguments passed to the
initializer. Should initializer 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">BrokenProcessPool</code></a>,
as well as any attempt to submit more jobs to the pool.
<div class="versionchanged">
Changed in version 3.3: When one of the worker processes terminates abruptly, a
<code class="xref py py-exc docutils literal notranslate">BrokenProcessPool</code> error is now raised. Previously, behaviour
was undefined but operations on the executor or its futures would often
freeze or deadlock.
</div>
<div class="versionchanged">
Changed in version 3.7: The mp_context argument was added to allow users to control the
start_method for worker processes created by the pool.
Added the initializer and initargs arguments.
</div>
</dd></dl>

<section id="processpoolexecutor-example">
<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>import concurrent.futures
import math

PRIMES = [
 112272535095293,
 112582705942171,
 112272535095293,
 115280095190773,
 115797848077099,
 1099726899285419]

def is_prime(n):
 if n &lt; 2:
 return False
 if n == 2:
 return True
 if n % 2 == 0:
 return False

sqrt_n = int(math.floor(math.sqrt(n)))
 for i in range(3, sqrt_n + 1, 2):
 if n % i == 0:
 return False
 return True

def main():
 with concurrent.futures.ProcessPoolExecutor() as executor:
 for number, prime in zip(PRIMES, executor.map(is_prime, PRIMES)):
 print(&#39;%d is prime: %s&#39; % (number, prime))

if __name__ == &#39;__main__&#39;:
 main()
</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>
The <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate">Future</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">Future</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">Executor.submit()</code></a>.
<dl class="py class">
<dt class="sig sig-object py" id="concurrent.futures.Future">
class concurrent.futures.Future<a class="headerlink" href="#concurrent.futures.Future" title="Permalink to this definition">¶</a></dt>
<dd>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">Future</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">Executor.submit()</code></a> and should not be created
directly except for testing.
<blockquote>
<div><dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.cancel">
cancel()<a class="headerlink" href="#concurrent.futures.Future.cancel" title="Permalink to this definition">¶</a></dt>
<dd>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">False</code>, otherwise the call will be cancelled and the method will
return <code class="docutils literal notranslate">True</code>.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.cancelled">
cancelled()<a class="headerlink" href="#concurrent.futures.Future.cancelled" title="Permalink to this definition">¶</a></dt>
<dd>Return <code class="docutils literal notranslate">True</code> if the call was successfully cancelled.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.running">
running()<a class="headerlink" href="#concurrent.futures.Future.running" title="Permalink to this definition">¶</a></dt>
<dd>Return <code class="docutils literal notranslate">True</code> if the call is currently being executed and cannot be
cancelled.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.done">
done()<a class="headerlink" href="#concurrent.futures.Future.done" title="Permalink to this definition">¶</a></dt>
<dd>Return <code class="docutils literal notranslate">True</code> if the call was successfully cancelled or finished
running.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.result">
result(timeout=None)<a class="headerlink" href="#concurrent.futures.Future.result" title="Permalink to this definition">¶</a></dt>
<dd>Return the value returned by the call. If the call hasn’t yet completed
then this method will wait up to timeout seconds. If the call hasn’t
completed in timeout seconds, then a
<a class="reference internal" href="#concurrent.futures.TimeoutError" title="concurrent.futures.TimeoutError"><code class="xref py py-exc docutils literal notranslate">concurrent.futures.TimeoutError</code></a> will be raised. timeout can be
an int or float. If timeout is not specified or <code class="docutils literal notranslate">None</code>, there is no
limit to the wait time.
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">CancelledError</code></a>
will be raised.
If the call raised an exception, this method will raise the same exception.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.exception">
exception(timeout=None)<a class="headerlink" href="#concurrent.futures.Future.exception" title="Permalink to this definition">¶</a></dt>
<dd>Return the exception raised by the call. If the call hasn’t yet
completed then this method will wait up to timeout seconds. If the
call hasn’t completed in timeout seconds, then a
<a class="reference internal" href="#concurrent.futures.TimeoutError" title="concurrent.futures.TimeoutError"><code class="xref py py-exc docutils literal notranslate">concurrent.futures.TimeoutError</code></a> will be raised. timeout can be
an int or float. If timeout is not specified or <code class="docutils literal notranslate">None</code>, there is no
limit to the wait time.
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">CancelledError</code></a>
will be raised.
If the call completed without raising, <code class="docutils literal notranslate">None</code> is returned.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.add_done_callback">
add_done_callback(fn)<a class="headerlink" href="#concurrent.futures.Future.add_done_callback" title="Permalink to this definition">¶</a></dt>
<dd>Attaches the callable fn to the future. fn will be called, with the
future as its only argument, when the future is cancelled or finishes
running.
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">Exception</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">BaseException</code></a> subclass, the
behavior is undefined.
If the future has already completed or been cancelled, fn will be
called immediately.
</dd></dl>

</div></blockquote>
The following <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate">Future</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">Executor</code></a> implementations.
<blockquote>
<div><dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.set_running_or_notify_cancel">
set_running_or_notify_cancel()<a class="headerlink" href="#concurrent.futures.Future.set_running_or_notify_cancel" title="Permalink to this definition">¶</a></dt>
<dd>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">Executor</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">Future</code></a> and by unit
tests.
If the method returns <code class="docutils literal notranslate">False</code> then the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate">Future</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">Future.cancel()</code></a> was called and returned <code class="docutils literal notranslate">True</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">Future</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">as_completed()</code></a> or <a class="reference internal" href="#concurrent.futures.wait" title="concurrent.futures.wait"><code class="xref py py-func docutils literal notranslate">wait()</code></a>) will be woken up.
If the method returns <code class="docutils literal notranslate">True</code> then the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate">Future</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">Future.running()</code></a> will return <code class="docutils literal notranslate">True</code>.
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">Future.set_result()</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">Future.set_exception()</code></a> have been
called.
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.set_result">
set_result(result)<a class="headerlink" href="#concurrent.futures.Future.set_result" title="Permalink to this definition">¶</a></dt>
<dd>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">Future</code></a> to
result.
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">Executor</code></a> implementations and
unit tests.
<div class="versionchanged">
Changed in version 3.8: This method raises
<a class="reference internal" href="#concurrent.futures.InvalidStateError" title="concurrent.futures.InvalidStateError"><code class="xref py py-exc docutils literal notranslate">concurrent.futures.InvalidStateError</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">Future</code></a> is
already done.
</div>
</dd></dl>

<dl class="py method">
<dt class="sig sig-object py" id="concurrent.futures.Future.set_exception">
set_exception(exception)<a class="headerlink" href="#concurrent.futures.Future.set_exception" title="Permalink to this definition">¶</a></dt>
<dd>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">Future</code></a> to the
<a class="reference internal" href="exceptions.html#Exception" title="Exception"><code class="xref py py-class docutils literal notranslate">Exception</code></a> exception.
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">Executor</code></a> implementations and
unit tests.
<div class="versionchanged">
Changed in version 3.8: This method raises
<a class="reference internal" href="#concurrent.futures.InvalidStateError" title="concurrent.futures.InvalidStateError"><code class="xref py py-exc docutils literal notranslate">concurrent.futures.InvalidStateError</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">Future</code></a> is
already done.
</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">
concurrent.futures.wait(fs, timeout=None, return_when=ALL_COMPLETED)<a class="headerlink" href="#concurrent.futures.wait" title="Permalink to this definition">¶</a></dt>
<dd>Wait for the <a class="reference internal" href="#concurrent.futures.Future" title="concurrent.futures.Future"><code class="xref py py-class docutils literal notranslate">Future</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">Executor</code></a> instances) given by fs to complete. Duplicate futures
given to fs are removed and will be returned only once. Returns a named
2-tuple of sets. The first set, named <code class="docutils literal notranslate">done</code>, contains the futures that
completed (finished or cancelled futures) before the wait completed. The
second set, named <code class="docutils literal notranslate">not_done</code>, contains the futures that did not complete
(pending or running futures).
timeout can be used to control the maximum number of seconds to wait before
returning. timeout can be an int or float. If timeout is not specified
or <code class="docutils literal notranslate">None</code>, there is no limit to the wait time.
return_when indicates when this function should return. It must be one of
the following constants:
<table class="docutils align-default">
<colgroup>
<col style="width: 42%" />
<col style="width: 58%" />
</colgroup>
<thead>
<tr class="row-odd"><th class="head">Constant</th>
<th class="head">Description</th>
</tr>
</thead>
<tbody>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">FIRST_COMPLETED</code></td>
<td>The function will return when any
future finishes or is cancelled.</td>
</tr>
<tr class="row-odd"><td><code class="xref py py-const docutils literal notranslate">FIRST_EXCEPTION</code></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
<code class="xref py py-const docutils literal notranslate">ALL_COMPLETED</code>.</td>
</tr>
<tr class="row-even"><td><code class="xref py py-const docutils literal notranslate">ALL_COMPLETED</code></td>
<td>The function will return when all
futures finish or are cancelled.</td>
</tr>
</tbody>
</table>
</dd></dl>

<dl class="py function">
<dt class="sig sig-object py" id="concurrent.futures.as_completed">
concurrent.futures.as_completed(fs, timeout=None)<a class="headerlink" href="#concurrent.futures.as_completed" title="Permalink to this definition">¶</a></dt>
<dd>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">Future</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">Executor</code></a> instances) given by fs that yields futures as
they complete (finished or cancelled futures). Any futures given by fs 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">as_completed()</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">concurrent.futures.TimeoutError</code></a> if <a class="reference internal" href="stdtypes.html#iterator.__next__" title="iterator.__next__"><code class="xref py py-meth docutils literal notranslate">__next__()</code></a>
is called and the result isn’t available after timeout 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">as_completed()</code></a>. timeout can be an int or float. If
timeout is not specified or <code class="docutils literal notranslate">None</code>, there is no limit to the wait time.
</dd></dl>

<div class="admonition seealso">
See also
<dl class="simple">
<dt><a class="pep reference external" href="https://www.python.org/dev/peps/pep-3148">PEP 3148</a> – futures - execute computations asynchronously</dt><dd>The proposal which described this feature for inclusion in the Python
standard library.
</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">
exception concurrent.futures.CancelledError<a class="headerlink" href="#concurrent.futures.CancelledError" title="Permalink to this definition">¶</a></dt>
<dd>Raised when a future is cancelled.
</dd></dl>

<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.TimeoutError">
exception concurrent.futures.TimeoutError<a class="headerlink" href="#concurrent.futures.TimeoutError" title="Permalink to this definition">¶</a></dt>
<dd>Raised when a future operation exceeds the given timeout.
</dd></dl>

<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.BrokenExecutor">
exception concurrent.futures.BrokenExecutor<a class="headerlink" href="#concurrent.futures.BrokenExecutor" title="Permalink to this definition">¶</a></dt>
<dd>Derived from <a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate">RuntimeError</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.
<div class="versionadded">
New in version 3.7.
</div>
</dd></dl>

<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.InvalidStateError">
exception concurrent.futures.InvalidStateError<a class="headerlink" href="#concurrent.futures.InvalidStateError" title="Permalink to this definition">¶</a></dt>
<dd>Raised when an operation is performed on a future that is not allowed
in the current state.
<div class="versionadded">
New in version 3.8.
</div>
</dd></dl>

<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.thread.BrokenThreadPool">
exception concurrent.futures.thread.BrokenThreadPool<a class="headerlink" href="#concurrent.futures.thread.BrokenThreadPool" title="Permalink to this definition">¶</a></dt>
<dd>Derived from <a class="reference internal" href="#concurrent.futures.BrokenExecutor" title="concurrent.futures.BrokenExecutor"><code class="xref py py-exc docutils literal notranslate">BrokenExecutor</code></a>, this exception
class is raised when one of the workers of a <code class="xref py py-class docutils literal notranslate">ThreadPoolExecutor</code>
has failed initializing.
<div class="versionadded">
New in version 3.7.
</div>
</dd></dl>

<dl class="py exception">
<dt class="sig sig-object py" id="concurrent.futures.process.BrokenProcessPool">
exception concurrent.futures.process.BrokenProcessPool<a class="headerlink" href="#concurrent.futures.process.BrokenProcessPool" title="Permalink to this definition">¶</a></dt>
<dd>Derived from <a class="reference internal" href="#concurrent.futures.BrokenExecutor" title="concurrent.futures.BrokenExecutor"><code class="xref py py-exc docutils literal notranslate">BrokenExecutor</code></a> (formerly
<a class="reference internal" href="exceptions.html#RuntimeError" title="RuntimeError"><code class="xref py py-exc docutils literal notranslate">RuntimeError</code></a>), this exception class is raised when one of the
workers of a <code class="xref py py-class docutils literal notranslate">ProcessPoolExecutor</code> has terminated in a non-clean
fashion (for example, if it was killed from the outside).
<div class="versionadded">
New in version 3.3.
</div>
</dd></dl>

</section>
</section>

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 <ul>
<li><a class="reference internal" href="#"><code class="xref py py-mod docutils literal notranslate">concurrent.futures</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>
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