.. 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')

::

   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.

.. class:: ProcessPoolExecutor(max_workers=None, mp_context=None, initializer=None, initargs=())

   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.
   *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:

ProcessPoolExecutor Example

   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

.. method:: cancel()

   Attempt to cancel the call.  If the call is currently being executed 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.

.. 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.