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+:mod:`concurrent.futures` --- Asynchronous computation
+======================================================
+
+.. module:: concurrent.futures
+ :synopsis: Execute computations asynchronously 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`.
+
+.. method:: Executor.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:: Executor.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:`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:: Executor.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 Objects
+--------------------------
+
+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)
+
+ Executes calls asynchronously using at pool of at most *max_workers* threads.
+
+.. _threadpoolexecutor-example:
+
+ThreadPoolExecutor Example
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+::
+
+ from concurrent import 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 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 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 Objects
+---------------------------
+
+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)
+
+ 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 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 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`.
+
+.. method:: Future.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:: Future.cancelled()
+
+ Return `True` if the call was successfully cancelled.
+
+.. method:: Future.running()
+
+ Return `True` if the call is currently being executed and cannot be
+ cancelled.
+
+.. method:: Future.done()
+
+ Return `True` if the call was successfully cancelled or finished running.
+
+.. method:: Future.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:: Future.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:: Future.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.
+
+Internal Future Methods
+^^^^^^^^^^^^^^^^^^^^^^^
+
+The following :class:`Future` methods are meant for use in unit tests and
+:class:`Executor` implementations.
+
+.. method:: Future.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:: Future.set_result(result)
+
+ Sets the result of the work associated with the :class:`Future` to *result*.
+
+ This method should only be used by Executor implementations and unit tests.
+
+.. method:: Future.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 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.