concurrent.futures — Asynchronous computation¶
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.
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Executor.submit(fn, *args, **kwargs)¶ Schedules the callable to be executed as fn*(*args*, **kwargs) and returns a
Futurerepresenting the execution of the callable.
with ThreadPoolExecutor(max_workers=1) as executor:
future = executor.submit(pow, 323, 1235)
print(future.result())
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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
TimeoutErrorif__next__()is called and the result isn’t available after timeout seconds from the original call tomap(). timeout can be an int or float. If timeout is not specified orNonethen 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.
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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()andExecutor.map()made after shutdown will raiseRuntimeError.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)
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class
concurrent.futures.ThreadPoolExecutor(max_workers)¶ 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.
-
class
concurrent.futures.ProcessPoolExecutor(max_workers=None)¶ Executes calls asynchronously using a pool of at most max_workers processes. If max_workers is
Noneor 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.
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Future.cancelled()¶ Return True if the call was successfully cancelled.
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Future.running()¶ Return True if the call is currently being executed and cannot be cancelled.
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Future.done()¶ Return True if the call was successfully cancelled or finished running.
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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
TimeoutErrorwill be raised. timeout can be an int or float.If timeout is not specified orNonethen there is no limit to the wait time.If the future is cancelled before completing then
CancelledErrorwill 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
TimeoutErrorwill be raised. timeout can be an int or float. If timeout is not specified orNonethen there is no limit to the wait time.If the future is cancelled before completing then
CancelledErrorwill be raised.If the call completed without raising then
Noneis returned.
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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
Exceptionthen it will be logged and ignored. If the callable raises anotherBaseExceptionthen 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.
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Future.set_running_or_notify_cancel()¶ This method should only be called by
Executorimplementations before executing the work associated with theFutureand by unit tests.If the method returns False then the
Futurewas cancelled i.e.Future.cancel()was called and returned True. Any threads waiting on theFuturecompleting (i.e. throughas_completed()orwait()) will be woken up.If the method returns True then the
Futurewas not cancelled and has been put in the running state i.e. calls toFuture.running()will return True.This method can only be called once and cannot be called after
Future.set_result()orFuture.set_exception()have been called.
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Future.set_result(result)¶ Sets the result of the work associated with the
Futureto 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
Futureto theExceptionexception.This method should only be used by Executor implementations and unit tests.
Module Functions¶
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concurrent.futures.wait(fs, timeout=None, return_when=ALL_COMPLETED)¶ Wait for the
Futureinstances (possibly created by differentExecutorinstances) 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
Nonethen 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_COMPLETEDThe function will return when any future finishes or is cancelled. FIRST_EXCEPTIONThe 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_COMPLETEDThe function will return when all futures finish or are cancelled.
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concurrent.futures.as_completed(fs, timeout=None)¶ Returns an iterator over the
Futureinstances (possibly created by differentExecutorinstances) 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 beforeas_completed()is called will be yielded first. The returned iterator raises aTimeoutErrorif__next__()is called and the result isn’t available after timeout seconds from the original call toas_completed(). timeout can be an int or float. If timeout is not specified orNone, there is no limit to the wait time.