cpython/Doc/library/concurrent.futures.rst

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:mod:`concurrent.futures` --- Launching parallel tasks
======================================================
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.. module:: concurrent.futures
:synopsis: Execute computations concurrently using threads or processes.
.. versionadded:: 3.2
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**Source code:** :source:`Lib/concurrent/futures/thread.py`
and :source:`Lib/concurrent/futures/process.py`
--------------
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The :mod:`concurrent.futures` module provides a high-level interface for
asynchronously executing callables.
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The asynchronous execution can be performed with threads, using
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:class:`ThreadPoolExecutor`, or separate processes, using
:class:`ProcessPoolExecutor`. Both implement the same interface, which is
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defined by the abstract :class:`Executor` class.
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Executor Objects
----------------
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.. class:: Executor
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An abstract class that provides methods to execute calls asynchronously. It
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should not be used directly, but through its concrete subclasses.
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.. method:: submit(fn, *args, **kwargs)
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Schedules the callable, *fn*, to be executed as ``fn(*args **kwargs)``
and returns a :class:`Future` object representing the execution of the
callable. ::
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with ThreadPoolExecutor(max_workers=1) as executor:
future = executor.submit(pow, 323, 1235)
print(future.result())
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.. method:: map(func, *iterables, timeout=None)
Equivalent to :func:`map(func, *iterables) <map>` except *func* is executed
asynchronously and several calls to *func* may be made concurrently. The
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returned iterator raises a :exc:`TimeoutError` if
:meth:`~iterator.__next__` is called and the result isn't available
after *timeout* seconds from the original call to :meth:`Executor.map`.
*timeout* can be an int or a float. If *timeout* is not specified or
``None``, 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.
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.. method:: shutdown(wait=True)
Signal the executor that it should free any resources that it is using
when the currently pending futures are done executing. Calls to
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:meth:`Executor.submit` and :meth:`Executor.map` made after shutdown will
raise :exc:`RuntimeError`.
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If *wait* is ``True`` then this method will not return until all the
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pending futures are done executing and the resources associated with the
executor have been freed. If *wait* is ``False`` then this method will
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return immediately and the resources associated with the executor will be
freed when all pending futures are done executing. Regardless of the
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value of *wait*, the entire Python program will not exit until all
pending futures are done executing.
You can avoid having to call this method explicitly if you use the
:keyword:`with` statement, which will shutdown the :class:`Executor`
(waiting as if :meth:`Executor.shutdown` were called with *wait* set to
``True``)::
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import shutil
with ThreadPoolExecutor(max_workers=4) as e:
e.submit(shutil.copy, 'src1.txt', 'dest1.txt')
e.submit(shutil.copy, 'src2.txt', 'dest2.txt')
e.submit(shutil.copy, 'src3.txt', 'dest3.txt')
e.submit(shutil.copy, 'src3.txt', 'dest4.txt')
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ThreadPoolExecutor
------------------
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:class:`ThreadPoolExecutor` is a :class:`Executor` subclass that uses a pool of
threads to execute calls asynchronously.
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Deadlocks can occur when the callable associated with a :class:`Future` waits on
the results of another :class:`Future`. For example::
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import time
def wait_on_b():
time.sleep(5)
print(b.result()) # b will never complete because it is waiting on a.
return 5
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def wait_on_a():
time.sleep(5)
print(a.result()) # a will never complete because it is waiting on b.
return 6
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executor = ThreadPoolExecutor(max_workers=2)
a = executor.submit(wait_on_b)
b = executor.submit(wait_on_a)
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And::
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def wait_on_future():
f = executor.submit(pow, 5, 2)
# This will never complete because there is only one worker thread and
# it is executing this function.
print(f.result())
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executor = ThreadPoolExecutor(max_workers=1)
executor.submit(wait_on_future)
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.. class:: ThreadPoolExecutor(max_workers)
An :class:`Executor` subclass that uses a pool of at most *max_workers*
threads to execute calls asynchronously.
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.. _threadpoolexecutor-example:
ThreadPoolExecutor Example
~~~~~~~~~~~~~~~~~~~~~~~~~~
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::
import concurrent.futures
import urllib.request
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URLS = ['http://www.foxnews.com/',
'http://www.cnn.com/',
'http://europe.wsj.com/',
'http://www.bbc.co.uk/',
'http://some-made-up-domain.com/']
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# Retrieve a single page and report the url and contents
def load_url(url, timeout):
conn = urllib.request.urlopen(url, timeout=timeout)
return conn.readall()
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# 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
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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('%r generated an exception: %s' % (url, exc))
else:
print('%r page is %d bytes' % (url, len(data)))
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ProcessPoolExecutor
-------------------
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The :class:`ProcessPoolExecutor` class is an :class:`Executor` subclass that
uses a pool of processes to execute calls asynchronously.
:class:`ProcessPoolExecutor` uses the :mod:`multiprocessing` module, which
allows it to side-step the :term:`Global Interpreter Lock` but also means that
only picklable objects can be executed and returned.
The ``__main__`` module must be importable by worker subprocesses. This means
that :class:`ProcessPoolExecutor` will not work in the interactive interpreter.
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Calling :class:`Executor` or :class:`Future` methods from a callable submitted
to a :class:`ProcessPoolExecutor` will result in deadlock.
.. class:: ProcessPoolExecutor(max_workers=None)
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An :class:`Executor` subclass that executes calls asynchronously using a pool
of at most *max_workers* processes. If *max_workers* is ``None`` or not
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given, it will default to the number of processors on the machine.
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.. versionchanged:: 3.3
When one of the worker processes terminates abruptly, a
:exc:`BrokenProcessPool` error is now raised. Previously, behaviour
was undefined but operations on the executor or its futures would often
freeze or deadlock.
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.. _processpoolexecutor-example:
ProcessPoolExecutor Example
~~~~~~~~~~~~~~~~~~~~~~~~~~~
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::
import concurrent.futures
import math
PRIMES = [
112272535095293,
112582705942171,
112272535095293,
115280095190773,
115797848077099,
1099726899285419]
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def is_prime(n):
if n % 2 == 0:
return False
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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
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def main():
with concurrent.futures.ProcessPoolExecutor() as executor:
for number, prime in zip(PRIMES, executor.map(is_prime, PRIMES)):
print('%d is prime: %s' % (number, prime))
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if __name__ == '__main__':
main()
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Future Objects
--------------
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The :class:`Future` class encapsulates the asynchronous execution of a callable.
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:class:`Future` instances are created by :meth:`Executor.submit`.
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.. class:: Future
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Encapsulates the asynchronous execution of a callable. :class:`Future`
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instances are created by :meth:`Executor.submit` and should not be created
directly except for testing.
.. method:: cancel()
Attempt to cancel the call. If the call is currently being executed and
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cannot be cancelled then the method will return ``False``, otherwise the
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call will be cancelled and the method will return ``True``.
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.. method:: cancelled()
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Return ``True`` if the call was successfully cancelled.
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.. method:: running()
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Return ``True`` if the call is currently being executed and cannot be
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cancelled.
.. method:: done()
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Return ``True`` if the call was successfully cancelled or finished
running.
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.. method:: result(timeout=None)
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
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completed in *timeout* seconds, then a :exc:`TimeoutError` will be
raised. *timeout* can be an int or float. If *timeout* is not specified
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or ``None``, there is no limit to the wait time.
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If the future is cancelled before completing then :exc:`CancelledError`
will be raised.
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If the call raised, this method will raise the same exception.
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.. method:: exception(timeout=None)
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 :exc:`TimeoutError`
will be raised. *timeout* can be an int or float. If *timeout* is not
specified or ``None``, there is no limit to the wait time.
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If the future is cancelled before completing then :exc:`CancelledError`
will be raised.
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If the call completed without raising, ``None`` is returned.
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.. method:: add_done_callback(fn)
Attaches the callable *fn* to the future. *fn* will be called, with the
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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
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the callable raises a :exc:`Exception` subclass, it will be logged and
ignored. If the callable raises a :exc:`BaseException` subclass, the
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behavior is undefined.
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If the future has already completed or been cancelled, *fn* will be
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called immediately.
The following :class:`Future` methods are meant for use in unit tests and
:class:`Executor` implementations.
.. method:: set_running_or_notify_cancel()
This method should only be called by :class:`Executor` implementations
before executing the work associated with the :class:`Future` and by unit
tests.
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If the method returns ``False`` then the :class:`Future` was cancelled,
i.e. :meth:`Future.cancel` was called and returned `True`. Any threads
waiting on the :class:`Future` completing (i.e. through
:func:`as_completed` or :func:`wait`) will be woken up.
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If the method returns ``True`` then the :class:`Future` was not cancelled
and has been put in the running state, i.e. calls to
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:meth:`Future.running` will return `True`.
This method can only be called once and cannot be called after
:meth:`Future.set_result` or :meth:`Future.set_exception` have been
called.
.. method:: set_result(result)
Sets the result of the work associated with the :class:`Future` to
*result*.
This method should only be used by :class:`Executor` implementations and
unit tests.
.. method:: set_exception(exception)
Sets the result of the work associated with the :class:`Future` to the
:class:`Exception` *exception*.
This method should only be used by :class:`Executor` implementations and
unit tests.
Module Functions
----------------
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.. function:: wait(fs, timeout=None, return_when=ALL_COMPLETED)
Wait for the :class:`Future` instances (possibly created by different
:class:`Executor` instances) given by *fs* to complete. Returns a named
2-tuple of sets. The first set, named ``done``, contains the futures that
completed (finished or were cancelled) before the wait completed. The second
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set, named ``not_done``, contains uncompleted futures.
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*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 ``None``, there is no limit to the wait time.
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*return_when* indicates when this function should return. It must be one of
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the following constants:
.. tabularcolumns:: |l|L|
+-----------------------------+----------------------------------------+
| Constant | Description |
+=============================+========================================+
| :const:`FIRST_COMPLETED` | The function will return when any |
| | future finishes or is cancelled. |
+-----------------------------+----------------------------------------+
| :const:`FIRST_EXCEPTION` | The function will return when any |
| | future finishes by raising an |
| | exception. If no future raises an |
| | exception then it is equivalent to |
| | :const:`ALL_COMPLETED`. |
+-----------------------------+----------------------------------------+
| :const:`ALL_COMPLETED` | The function will return when all |
| | futures finish or are cancelled. |
+-----------------------------+----------------------------------------+
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.. function:: as_completed(fs, timeout=None)
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Returns an iterator over the :class:`Future` instances (possibly created by
different :class:`Executor` instances) given by *fs* that yields futures as
they complete (finished or were cancelled). Any futures given by *fs* that
are duplicated will be returned once. Any futures that completed
before :func:`as_completed` is called will be yielded first. The returned
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iterator raises a :exc:`TimeoutError` if :meth:`~iterator.__next__` is
called and the result isn't available after *timeout* seconds from the
original call to :func:`as_completed`. *timeout* can be an int or float.
If *timeout* is not specified or ``None``, there is no limit to the wait
time.
.. seealso::
:pep:`3148` -- futures - execute computations asynchronously
The proposal which described this feature for inclusion in the Python
standard library.
Exception classes
-----------------
.. exception:: BrokenProcessPool
Derived from :exc:`RuntimeError`, this exception class is raised when
one of the workers of a :class:`ProcessPoolExecutor` has terminated
in a non-clean fashion (for example, if it was killed from the outside).
.. versionadded:: 3.3