Python’s `concurrent.futures`: Iterate on futures according to order of completion

executor.map(), like the builtin map(), only returns results in the order of the iterable, so unfortunately you can’t use it to determine the order of completion. concurrent.futures.as_completed() is what you’re looking for – here’s an example:

import time
import concurrent.futures

times = [3, 1, 2]

def sleeper(secs):
    time.sleep(secs)
    print('I slept for {} seconds'.format(secs))
    return secs

# returns in the order given
with concurrent.futures.ThreadPoolExecutor(max_workers=3) as executor:
    print(list(executor.map(sleeper, times)))

# I slept for 1 seconds
# I slept for 2 seconds
# I slept for 3 seconds
# [3, 1, 2]

# returns in the order completed
with concurrent.futures.ThreadPoolExecutor(max_workers=3) as executor:
    futs = [executor.submit(sleeper, secs) for secs in times]
    print([fut.result() for fut in concurrent.futures.as_completed(futs)])

# I slept for 1 seconds
# I slept for 2 seconds
# I slept for 3 seconds
# [1, 2, 3]

Of course if you are required to use a map interface, you could create your own map_as_completed() function which encapsulates the above (maybe add it to a subclassed Executor()), but I think creating futures instances through executor.submit() is a simpler/cleaner way to go (also allows you to provide no-args, kwargs).