346 lines
13 KiB
ReStructuredText
346 lines
13 KiB
ReStructuredText
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:mod:`concurrent.futures` --- Asynchronous computation
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======================================================
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.. module:: concurrent.futures
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:synopsis: Execute computations asynchronously using threads or processes.
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The :mod:`concurrent.futures` module provides a high-level interface for
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asynchronously executing callables.
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The asynchronous execution can be be performed by threads using
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:class:`ThreadPoolExecutor` or seperate processes using
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:class:`ProcessPoolExecutor`. Both implement the same interface, which is
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defined by the abstract :class:`Executor` class.
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Executor Objects
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----------------
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:class:`Executor` is an abstract class that provides methods to execute calls
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asynchronously. It should not be used directly, but through its two
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subclasses: :class:`ThreadPoolExecutor` and :class:`ProcessPoolExecutor`.
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.. method:: Executor.submit(fn, *args, **kwargs)
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Schedules the callable to be executed as *fn*(*\*args*, *\*\*kwargs*) and
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returns a :class:`Future` representing the execution of the callable.
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::
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with ThreadPoolExecutor(max_workers=1) as executor:
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future = executor.submit(pow, 323, 1235)
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print(future.result())
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.. method:: Executor.map(func, *iterables, timeout=None)
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Equivalent to map(*func*, *\*iterables*) but func is executed asynchronously
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and several calls to *func* may be made concurrently. The returned iterator
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raises a :exc:`TimeoutError` if :meth:`__next__()` is called and the result
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isn't available after *timeout* seconds from the original call to
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:meth:`map()`. *timeout* can be an int or float. If *timeout* is not
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specified or ``None`` then there is no limit to the wait time. If a call
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raises an exception then that exception will be raised when its value is
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retrieved from the iterator.
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.. method:: Executor.shutdown(wait=True)
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Signal the executor that it should free any resources that it is using when
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the currently pending futures are done executing. Calls to
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:meth:`Executor.submit` and :meth:`Executor.map` made after shutdown will
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raise :exc:`RuntimeError`.
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If *wait* is `True` then this method will not return until all the pending
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futures are done executing and the resources associated with the executor
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have been freed. If *wait* is `False` then this method will return
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immediately and the resources associated with the executor will be freed
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when all pending futures are done executing. Regardless of the value of
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*wait*, the entire Python program will not exit until all pending futures
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are done executing.
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You can avoid having to call this method explicitly if you use the `with`
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statement, which will shutdown the `Executor` (waiting as if
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`Executor.shutdown` were called with *wait* set to `True`):
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::
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import shutil
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with ThreadPoolExecutor(max_workers=4) as e:
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e.submit(shutil.copy, 'src1.txt', 'dest1.txt')
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e.submit(shutil.copy, 'src2.txt', 'dest2.txt')
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e.submit(shutil.copy, 'src3.txt', 'dest3.txt')
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e.submit(shutil.copy, 'src3.txt', 'dest4.txt')
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ThreadPoolExecutor Objects
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--------------------------
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The :class:`ThreadPoolExecutor` class is an :class:`Executor` subclass that uses
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a pool of threads to execute calls asynchronously.
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Deadlock can occur when the callable associated with a :class:`Future` waits on
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the results of another :class:`Future`. For example:
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::
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import time
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def wait_on_b():
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time.sleep(5)
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print(b.result()) # b will never complete because it is waiting on a.
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return 5
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def wait_on_a():
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time.sleep(5)
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print(a.result()) # a will never complete because it is waiting on b.
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return 6
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executor = ThreadPoolExecutor(max_workers=2)
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a = executor.submit(wait_on_b)
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b = executor.submit(wait_on_a)
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And:
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::
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def wait_on_future():
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f = executor.submit(pow, 5, 2)
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# This will never complete because there is only one worker thread and
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# it is executing this function.
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print(f.result())
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executor = ThreadPoolExecutor(max_workers=1)
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executor.submit(wait_on_future)
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.. class:: ThreadPoolExecutor(max_workers)
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Executes calls asynchronously using at pool of at most *max_workers* threads.
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.. _threadpoolexecutor-example:
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ThreadPoolExecutor Example
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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::
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from concurrent import futures
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import urllib.request
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URLS = ['http://www.foxnews.com/',
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'http://www.cnn.com/',
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'http://europe.wsj.com/',
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'http://www.bbc.co.uk/',
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'http://some-made-up-domain.com/']
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def load_url(url, timeout):
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return urllib.request.urlopen(url, timeout=timeout).read()
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with futures.ThreadPoolExecutor(max_workers=5) as executor:
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future_to_url = dict((executor.submit(load_url, url, 60), url)
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for url in URLS)
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for future in futures.as_completed(future_to_url):
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url = future_to_url[future]
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if future.exception() is not None:
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print('%r generated an exception: %s' % (url,
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future.exception()))
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else:
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print('%r page is %d bytes' % (url, len(future.result())))
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ProcessPoolExecutor Objects
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---------------------------
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The :class:`ProcessPoolExecutor` class is an :class:`Executor` subclass that
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uses a pool of processes to execute calls asynchronously.
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:class:`ProcessPoolExecutor` uses the :mod:`multiprocessing` module, which
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allows it to side-step the :term:`Global Interpreter Lock` but also means that
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only picklable objects can be executed and returned.
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Calling :class:`Executor` or :class:`Future` methods from a callable submitted
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to a :class:`ProcessPoolExecutor` will result in deadlock.
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.. class:: ProcessPoolExecutor(max_workers=None)
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Executes calls asynchronously using a pool of at most *max_workers*
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processes. If *max_workers* is ``None`` or not given then as many worker
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processes will be created as the machine has processors.
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.. _processpoolexecutor-example:
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ProcessPoolExecutor Example
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^^^^^^^^^^^^^^^^^^^^^^^^^^^
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::
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import math
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PRIMES = [
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112272535095293,
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112582705942171,
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112272535095293,
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115280095190773,
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115797848077099,
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1099726899285419]
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def is_prime(n):
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if n % 2 == 0:
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return False
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sqrt_n = int(math.floor(math.sqrt(n)))
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for i in range(3, sqrt_n + 1, 2):
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if n % i == 0:
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return False
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return True
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def main():
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with futures.ProcessPoolExecutor() as executor:
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for number, prime in zip(PRIMES, executor.map(is_prime, PRIMES)):
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print('%d is prime: %s' % (number, prime))
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if __name__ == '__main__':
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main()
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Future Objects
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--------------
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The :class:`Future` class encapulates the asynchronous execution of a callable.
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:class:`Future` instances are created by :meth:`Executor.submit`.
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.. method:: Future.cancel()
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Attempt to cancel the call. If the call is currently being executed then
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it cannot be cancelled and the method will return `False`, otherwise the call
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will be cancelled and the method will return `True`.
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.. method:: Future.cancelled()
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Return `True` if the call was successfully cancelled.
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.. method:: Future.running()
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Return `True` if the call is currently being executed and cannot be
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cancelled.
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.. method:: Future.done()
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Return `True` if the call was successfully cancelled or finished running.
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.. method:: Future.result(timeout=None)
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Return the value returned by the call. If the call hasn't yet completed then
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this method will wait up to *timeout* seconds. If the call hasn't completed
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in *timeout* seconds then a :exc:`TimeoutError` will be raised. *timeout* can
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be an int or float.If *timeout* is not specified or ``None`` then there is no
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limit to the wait time.
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If the future is cancelled before completing then :exc:`CancelledError` will
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be raised.
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If the call raised then this method will raise the same exception.
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.. method:: Future.exception(timeout=None)
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Return the exception raised by the call. If the call hasn't yet completed
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then this method will wait up to *timeout* seconds. If the call hasn't
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completed in *timeout* seconds then a :exc:`TimeoutError` will be raised.
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*timeout* can be an int or float. If *timeout* is not specified or ``None``
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then there is no limit to the wait time.
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If the future is cancelled before completing then :exc:`CancelledError` will
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be raised.
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If the call completed without raising then ``None`` is returned.
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.. method:: Future.add_done_callback(fn)
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Attaches the callable *fn* to the future. *fn* will be called, with the
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future as its only argument, when the future is cancelled or finishes
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running.
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Added callables are called in the order that they were added and are always
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called in a thread belonging to the process that added them. If the callable
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raises an :exc:`Exception` then it will be logged and ignored. If the
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callable raises another :exc:`BaseException` then the behavior is not
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defined.
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If the future has already completed or been cancelled then *fn* will be
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called immediately.
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Internal Future Methods
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^^^^^^^^^^^^^^^^^^^^^^^
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The following :class:`Future` methods are meant for use in unit tests and
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:class:`Executor` implementations.
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.. method:: Future.set_running_or_notify_cancel()
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This method should only be called by :class:`Executor` implementations before
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executing the work associated with the :class:`Future` and by unit tests.
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If the method returns `False` then the :class:`Future` was cancelled i.e.
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:meth:`Future.cancel` was called and returned `True`. Any threads waiting
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on the :class:`Future` completing (i.e. through :func:`as_completed` or
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:func:`wait`) will be woken up.
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If the method returns `True` then the :class:`Future` was not cancelled
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and has been put in the running state i.e. calls to
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:meth:`Future.running` will return `True`.
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This method can only be called once and cannot be called after
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:meth:`Future.set_result` or :meth:`Future.set_exception` have been
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called.
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.. method:: Future.set_result(result)
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Sets the result of the work associated with the :class:`Future` to *result*.
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This method should only be used by Executor implementations and unit tests.
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.. method:: Future.set_exception(exception)
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Sets the result of the work associated with the :class:`Future` to the
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:class:`Exception` *exception*.
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This method should only be used by Executor implementations and unit tests.
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Module Functions
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----------------
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.. function:: wait(fs, timeout=None, return_when=ALL_COMPLETED)
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Wait for the :class:`Future` instances (possibly created by different
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:class:`Executor` instances) given by *fs* to complete. Returns a named
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2-tuple of sets. The first set, named "done", contains the futures that
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completed (finished or were cancelled) before the wait completed. The second
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set, named "not_done", contains uncompleted futures.
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*timeout* can be used to control the maximum number of seconds to wait before
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returning. *timeout* can be an int or float. If *timeout* is not specified or
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``None`` then there is no limit to the wait time.
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*return_when* indicates when this function should return. It must be one of
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the following constants:
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+-----------------------------+----------------------------------------+
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| Constant | Description |
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+=============================+========================================+
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| :const:`FIRST_COMPLETED` | The function will return when any |
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| | future finishes or is cancelled. |
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+-----------------------------+----------------------------------------+
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| :const:`FIRST_EXCEPTION` | The function will return when any |
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| | future finishes by raising an |
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| | exception. If no future raises an |
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| | exception then it is equivalent to |
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| | `ALL_COMPLETED`. |
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+-----------------------------+----------------------------------------+
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| :const:`ALL_COMPLETED` | The function will return when all |
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| | futures finish or are cancelled. |
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+-----------------------------+----------------------------------------+
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.. function:: as_completed(fs, timeout=None)
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Returns an iterator over the :class:`Future` instances (possibly created
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by different :class:`Executor` instances) given by *fs* that yields futures
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as they complete (finished or were cancelled). Any futures that completed
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before :func:`as_completed()` was called will be yielded first. The returned
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iterator raises a :exc:`TimeoutError` if :meth:`__next__()` is called and
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the result isn't available after *timeout* seconds from the original call
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to :func:`as_completed()`. *timeout* can be an int or float. If *timeout*
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is not specified or ``None`` then there is no limit to the wait time.
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