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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 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:`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.