deb-python-concurrent.futures/docs/index.rst

concurrent.futures --- Asynchronous computation

concurrent.futures

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

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

Executor Objects

Executor is an abstract class that provides methods to execute calls asynchronously. It should not be used directly, but through its two subclasses: ThreadPoolExecutor and ProcessPoolExecutor.

Executor.submit(fn, args,*kwargs)

Schedules the callable to be executed as fn(*args, **kwargs) and returns a Future representing the execution of the callable.

with ThreadPoolExecutor(max_workers=1) as executor:
    future = executor.submit(pow, 323, 1235)
    print(future.result())

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 TimeoutError if __next__() is called and the result isn't available after timeout seconds from the original call to 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.

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 Executor.submit and Executor.map made after shutdown will raise 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 ThreadPoolExecutor class is an Executor subclass that uses a pool of threads to execute calls asynchronously.

Deadlock can occur when the callable associated with a Future waits on the results of another 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)

Executes calls asynchronously using a pool of at most max_workers threads.

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 ProcessPoolExecutor class is an Executor subclass that uses a pool of processes to execute calls asynchronously. ProcessPoolExecutor uses the multiprocessing module, which allows it to side-step the Global Interpreter Lock but also means that only picklable objects can be executed and returned.

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

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

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 Future class encapulates the asynchronous execution of a callable. Future instances are created by Executor.submit.

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.

Future.cancelled()

Return True if the call was successfully cancelled.

Future.running()

Return True if the call is currently being executed and cannot be cancelled.

Future.done()

Return True if the call was successfully cancelled or finished running.

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 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 CancelledError will be raised.

If the call raised then this method will raise the same exception.

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 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 CancelledError will be raised.

If the call completed without raising then None is returned.

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 Exception then it will be logged and ignored. If the callable raises another 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 Future methods are meant for use in unit tests and Executor implementations.

Future.set_running_or_notify_cancel()

This method should only be called by Executor implementations before executing the work associated with the Future and by unit tests.

If the method returns False then the Future was cancelled i.e. Future.cancel was called and returned True. Any threads waiting on the Future completing (i.e. through as_completed or wait) will be woken up.

If the method returns True then the Future was not cancelled and has been put in the running state i.e. calls to Future.running will return True.

This method can only be called once and cannot be called after Future.set_result or Future.set_exception have been called.

Future.set_result(result)

Sets the result of the work associated with the Future to result.

This method should only be used by Executor implementations and unit tests.

Future.set_exception(exception)

Sets the result of the work associated with the Future to the Exception exception.

This method should only be used by Executor implementations and unit tests.

Module Functions

wait(fs, timeout=None, return_when=ALL_COMPLETED)

Wait for the Future instances (possibly created by different 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
FIRST_COMPLETED	The function will return when any future finishes or is cancelled.
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.
ALL_COMPLETED	The function will return when all futures finish or are cancelled.

as_completed(fs, timeout=None)

Returns an iterator over the Future instances (possibly created by different 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 as_completed is called will be yielded first. The returned iterator raises a TimeoutError if ~iterator.__next__ is called and the result isn't available after timeout seconds from the original call to as_completed. timeout can be an int or float. If timeout is not specified or None, there is no limit to the wait time.