Executor

Executor is an abstract class that provides methods to execute calls asynchronously.

submit(fn, *args, **kwargs)

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

This is an abstract method and must be implemented by Executor subclasses.

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(). 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.

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 and 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.

When using an executor as a context manager, __exit__ will call Executor.shutdown(wait=True).

ProcessPoolExecutor

The ProcessPoolExecutor class is an Executor subclass that uses a pool of processes to execute calls asynchronously. The callable objects and arguments passed to ProcessPoolExecutor.submit must be pickleable according to the same limitations as the multiprocessing module.

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

__init__(max_workers)

Executes calls asynchronously using a pool of a 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.

ThreadPoolExecutor

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)

__init__(max_workers)

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

Future Objects

The Future class encapsulates the asynchronous execution of a callable. Future instances are returned by Executor.submit.

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.

cancelled()

Return True if the call was successfully cancelled.

running()

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

done()

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

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. 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.

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. 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.

add_done_callback(fn)

Attaches a callable fn to the future that will be called when the future is cancelled or finishes running. fn will be called with the future as its only argument.

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 behavior is not defined.

If the future has already completed or been cancelled then fn will be called immediately.

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. If timeout is not specified or None then there is no limit to the wait time.

return_when indicates when the method should return. It must be one of the following constants:

Constant	Description
FIRST_COMPLETED	The method will return when any future finishes or is cancelled.
FIRST_EXCEPTION	The method will return when any future finishes by raising an exception. If not future raises an exception then it is equivalent to ALL_COMPLETED.
ALL_COMPLETED	The method will return when all calls finish.

as_completed(fs, timeout=None)

Returns an iterator over the Future instances given by fs that yields futures as they complete (finished or were cancelled). Any futures that completed before as_completed() was called will be yielded first. The returned iterator raises a TimeoutError if __next__() is called and the result isn’t available after timeout seconds from the original call to as_completed(). If timeout is not specified or None then there is no limit to the wait time.

The Future instances can have been created by different Executor instances.