Get return value for multi-processing functions in python

Question

I have two functions to run in parallel and each of them returns a value. I need to wait for both functions to finish and then process the returns from them. How could I achieve this in python. Assume

def fun1():
   #do some calculation#
   return val1

def fun2():
   #do some calculation#
   return val2

I want fun1 and fun2 to run in parallel and then need to calculate

valsum = val1+val2

Asked By: TheStupidOne

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Source

Answer 1

Using concurrent.futures:

from concurrent.futures import ProcessPoolExecutor as Executor
#from concurrent.futures import ThreadPoolExecutor as Executor  # to use threads
with Executor() as executor:
    future1 = executor.submit(fun1, arg1, arg2, ...)
    future2 = executor.submit(fun2, arg1, arg2, ...)
    val1, val2 = future1.result(), future2.result()
    valsum = val1 + val2

concurrent.futures.Executor.submit schedules the function to be executed, and returns a concurrent.futures.Future object.

concurrent.futures.Future.resultreturns the return value returned by the function.

Using multiprocessing.pool:

from multiprocessing.pool import Pool
#from multiprocessing.pool import ThreadPool as Pool  # to use threads
with multiprocessing.pool.Pool() as pool:
    result1 = pool.apply_async(fun1, (arg1, arg2, ...))
    result2 = pool.apply_async(fun2, (arg1, arg2, ...))
    val1, val2 = result1.get(), result2.get()
    valsum = val1 + val2

multiprocessing.pool.Pool.apply_async returns an AsyncResult object, and
AsyncResult.get will return the return value of the function once it finish and the result arrive.

Answered By: falsetru

Answer 2

This is a simple example using list comprehension and calling .result() on each future which blocks until the process returns:

import concurrent.futures

def download_video(data):
    # print(data) 
    return data

with concurrent.futures.ProcessPoolExecutor() as executor:
    futures = [executor.submit(download_video, x, ) for x in your_list]
    results = [result.result() for result in futures]

Answered By: grantr

Get return value for multi-processing functions in python

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