Doc/library/concurrent.futures.rst

:mod:`concurrent.futures` --- Concurrent computation
====================================================

.. module:: concurrent.futures
   :synopsis: Execute computations concurrently using threads or processes.

The :mod:`concurrent.futures` module provides a high-level interface for
asynchronously executing callables.

The asynchronous execution can be be performed by threads using
:class:`ThreadPoolExecutor` or seperate processes using
:class:`ProcessPoolExecutor`. Both implement the same interface, which is
defined by the abstract :class:`Executor` class.

Executor Objects
^^^^^^^^^^^^^^^^

:class:`Executor` is an abstract class that provides methods to execute calls
asynchronously. It should not be used directly, but through its two
subclasses: :class:`ThreadPoolExecutor` and :class:`ProcessPoolExecutor`.

.. class:: Executor()

   An abstract class that provides methods to execute calls asynchronously. It
   should not be used directly, but through its two subclasses:
   :class:`ThreadPoolExecutor` and :class:`ProcessPoolExecutor`.

    .. method:: submit(fn, *args, **kwargs)

       Schedules the callable to be executed as *fn*(*\*args*, *\*\*kwargs*) and
       returns a :class:`Future` representing the execution of the callable.

       ::

        with ThreadPoolExecutor(max_workers=1) as executor:
            future = executor.submit(pow, 323, 1235)
            print(future.result())

    .. method:: map(func, *iterables, timeout=None)

       Equivalent to `map(*func*, *\*iterables*)` but func is executed
       asynchronously and several calls to *func* may be made concurrently. The
       returned iterator raises a :exc:`TimeoutError` if :meth:`__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
       float. If *timeout* is not specified or ``None`` then 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.

    .. 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
       :meth:`Executor.submit` and :meth:`Executor.map` made after shutdown will
       raise :exc:`RuntimeError`.

       If *wait* is `True` 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 `False` 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.

       You can avoid having to call this method explicitly if you use the `with`
       statement, which will shutdown the `Executor` (waiting as if
       `Executor.shutdown` were called with *wait* set to `True`):

       ::

        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')

ThreadPoolExecutor
^^^^^^^^^^^^^^^^^^

The :class:`ThreadPoolExecutor` class is an :class:`Executor` subclass that uses
a pool of threads to execute calls asynchronously.

Deadlock can occur when the callable associated with a :class:`Future` waits on
the results of another :class:`Future`. For example:

::

    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=2)
    a = executor.submit(wait_on_b)
    b = executor.submit(wait_on_a)

And:

::

    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())

    executor = ThreadPoolExecutor(max_workers=1)
    executor.submit(wait_on_future)


.. class:: ThreadPoolExecutor(max_workers)

   An :class:`Executor` subclass that uses a pool of at most *max_workers*
   threads to execute calls asynchronously.

   Deadlock can occur when the callable associated with a :class:`Future` waits
   on the results of another :class:`Future`.

.. _threadpoolexecutor-example:

ThreadPoolExecutor Example
^^^^^^^^^^^^^^^^^^^^^^^^^^
::

    import concurrent.futures
    import urllib.request

    URLS = ['http://www.foxnews.com/',
            'http://www.cnn.com/',
            'http://europe.wsj.com/',
            'http://www.bbc.co.uk/',
            'http://some-made-up-domain.com/']

    def load_url(url, timeout):
        return urllib.request.urlopen(url, timeout=timeout).read()

    with concurrent.futures.ThreadPoolExecutor(max_workers=5) as executor:
        future_to_url = dict((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]
            if future.exception() is not None:
                print('%r generated an exception: %s' % (url,
                                                         future.exception()))
            else:
                print('%r page is %d bytes' % (url, len(future.result())))


ProcessPoolExecutor
^^^^^^^^^^^^^^^^^^^

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.

Calling :class:`Executor` or :class:`Future` methods from a callable submitted
to a :class:`ProcessPoolExecutor` will result in deadlock.

.. class:: ProcessPoolExecutor(max_workers=None)

   An :class:`Executor` subclass that executes calls asynchronously using a
   pool of at most *max_workers* processes. If *max_workers* is ``None`` or
   not given then as many worker processes will be created as the machine has
   processors.

.. _processpoolexecutor-example:

ProcessPoolExecutor Example
^^^^^^^^^^^^^^^^^^^^^^^^^^^
::

    import concurrent.futures
    import math

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

    def is_prime(n):
        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('%d is prime: %s' % (number, prime))

    if __name__ == '__main__':
        main()

Future Objects
^^^^^^^^^^^^^^

The :class:`Future` class encapulates the asynchronous execution of a callable.
:class:`Future` instances are created by :meth:`Executor.submit`.

.. class:: Future()

   Encapulates the asynchronous execution of a callable. :class:`Future`
   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 then
       it cannot be cancelled and the method will return `False`, otherwise the
       call will be cancelled and the method will return `True`.

    .. method:: cancelled()

       Return `True` if the call was successfully cancelled.

    .. method:: running()

       Return `True` if the call is currently being executed and cannot be
       cancelled.

    .. method:: done()

       Return `True` if the call was successfully cancelled or finished running.

    .. 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
       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`` then there is no limit to the wait time.

       If the future is cancelled before completing then :exc:`CancelledError`
       will be raised.

       If the call raised then this method will raise the same exception.

    .. 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`` then there is no limit to the wait time.

       If the future is cancelled before completing then :exc:`CancelledError`
       will be raised.

       If the call completed without raising then ``None`` is returned.

    .. method:: add_done_callback(fn)

       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 :exc:`Exception` then it will be logged and
       ignored. If the callable raises another :exc:`BaseException` then the
       behavior is not defined.

       If the future has already completed or been cancelled then *fn* will be
       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.

       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.

       If the method returns `True` then the :class:`Future` was not cancelled
       and has been put in the running state i.e. calls to
       :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
^^^^^^^^^^^^^^^^

.. 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
   set, named "not_done", contains uncompleted 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
   ``None`` then there is no limit to the wait time.

   *return_when* indicates when this function should return. It must be one of
   the following constants:

      +-----------------------------+----------------------------------------+
      | 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     |
      |                             | `ALL_COMPLETED`.                       |
      +-----------------------------+----------------------------------------+
      | :const:`ALL_COMPLETED`      | The function will return when all      |
      |                             | futures finish or are cancelled.       |
      +-----------------------------+----------------------------------------+

.. function:: as_completed(fs, timeout=None)

   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 that completed
   before :func:`as_completed()` was called will be yielded first. The returned
   iterator raises a :exc:`TimeoutError` if :meth:`__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`` then there is no limit to the wait time.