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:mod:`concurrent.futures` --- Launching parallel tasks

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

.. versionadded:: 3.2

Source code: :source:`Lib/concurrent/futures/thread.py` and :source:`Lib/concurrent/futures/process.py`


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

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

Executor Objects

An abstract class that provides methods to execute calls asynchronously. It should not be used directly, but through its concrete subclasses.

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

   Schedules the callable, *fn*, to be executed as ``fn(*args **kwargs)``
   and returns a :class:`Future` object 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, chunksize=1)

   Similar to :func:`map(func, *iterables) <map>` except:

   * the *iterables* are collected immediately rather than lazily;

   * *func* is executed asynchronously and several calls to
     *func* may be made concurrently.

   The returned iterator raises a :exc:`concurrent.futures.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 *func* call raises an exception, then that exception will be
   raised when its value is retrieved from the iterator.

   When using :class:`ProcessPoolExecutor`, 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
   :class:`ThreadPoolExecutor`, *chunksize* has no effect.

   .. versionchanged:: 3.5
      Added the *chunksize* argument.

.. 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
   :keyword:`with` statement, which will shutdown the :class:`Executor`
   (waiting as if :meth:`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, 'src4.txt', 'dest4.txt')

ThreadPoolExecutor

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

Deadlocks 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)

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

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 :exc:`~concurrent.futures.thread.BrokenThreadPool`, as well as any attempt to submit more jobs to the pool.

.. versionchanged:: 3.5
   If *max_workers* is ``None`` or
   not given, it will default to the number of processors on the machine,
   multiplied by ``5``, assuming that :class:`ThreadPoolExecutor` 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 :class:`ProcessPoolExecutor`.

.. versionadded:: 3.6
   The *thread_name_prefix* argument was added to allow users to
   control the :class:`threading.Thread` names for worker threads created by
   the pool for easier debugging.

.. versionchanged:: 3.7
   Added the *initializer* and *initargs* arguments.

.. versionchanged:: 3.8
   Default value of *max_workers* is changed to ``min(32, os.cpu_count() + 4)``.
   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.

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/']

# 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('%r generated an exception: %s' % (url, exc))
        else:
            print('%r page is %d bytes' % (url, len(data)))

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.

The __main__ module must be importable by worker subprocesses. This means that :class:`ProcessPoolExecutor` will not work in the interactive interpreter.

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

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, it will default to the number of processors on the machine. If max_workers is lower or equal to 0, then a :exc:`ValueError` will be raised. On Windows, max_workers must be equal or lower than 61. If it is not then :exc:`ValueError` will be raised. If max_workers is None, then the default chosen will be at most 61, 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 None 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 :exc:`~concurrent.futures.process.BrokenProcessPool`, as well any attempt to submit more jobs to the pool.

.. 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.

.. versionchanged:: 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.

ProcessPoolExecutor Example

import concurrent.futures
import math

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

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

if __name__ == '__main__':
    main()

Future Objects

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

Encapsulates 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 or
   finished running and cannot be cancelled then 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:`concurrent.futures.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.

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

   If the call raised, 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:`concurrent.futures.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.

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

   If the call completed without raising, ``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` subclass, it will be logged and
   ignored.  If the callable raises a :exc:`BaseException` subclass, the
   behavior is undefined.

   If the future has already completed or been cancelled, *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.

   .. versionchanged:: 3.8
      This method raises
      :exc:`concurrent.futures.InvalidStateError` if the :class:`Future` is
      already done.

.. 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.

   .. versionchanged:: 3.8
      This method raises
      :exc:`concurrent.futures.InvalidStateError` if the :class:`Future` is
      already done.

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 cancelled futures) before the wait completed.  The
   second set, named ``not_done``, 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 ``None``, there is no limit to the wait time.

   *return_when* indicates when this function should return.  It must be one of
   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.       |
   +-----------------------------+----------------------------------------+

.. 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 cancelled futures). 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 iterator
   raises a :exc:`concurrent.futures.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

.. currentmodule:: concurrent.futures

.. exception:: CancelledError

   Raised when a future is cancelled.

.. exception:: TimeoutError

   Raised when a future operation exceeds the given timeout.

.. exception:: BrokenExecutor

   Derived from :exc:`RuntimeError`, this exception class is raised
   when an executor is broken for some reason, and cannot be used
   to submit or execute new tasks.

   .. versionadded:: 3.7

.. exception:: InvalidStateError

   Raised when an operation is performed on a future that is not allowed
   in the current state.

   .. versionadded:: 3.8

.. currentmodule:: concurrent.futures.thread

.. exception:: BrokenThreadPool

   Derived from :exc:`~concurrent.futures.BrokenExecutor`, this exception
   class is raised when one of the workers of a :class:`ThreadPoolExecutor`
   has failed initializing.

   .. versionadded:: 3.7

.. currentmodule:: concurrent.futures.process

.. exception:: BrokenProcessPool

   Derived from :exc:`~concurrent.futures.BrokenExecutor` (formerly
   :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
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