How to Retrieve Return Values from multiprocessing.Process Functions?

How to Retrieve Return Values from Functions Passed to multiprocessing.Process

When passing a function to multiprocessing.Process, there's often a need to retrieve the function's return value. However, these values are not readily accessible in the returned Process objects.

Consider the following code:

import multiprocessing

def worker(procnum):

'''worker function'''
print str(procnum) + ' represent!'
return procnum

if name == '__main__':

jobs = []
for i in range(5):
    p = multiprocessing.Process(target=worker, args=(i,))
    jobs.append(p)
    p.start()

for proc in jobs:
    proc.join()
print jobs

The expected return values are not available through the Process objects in 'jobs'.

Solution: Using a Shared Variable

To retrieve the return values, a shared variable can be utilized to enable communication between the main process and worker processes.

import multiprocessing

def worker(procnum, return_dict):

"""worker function"""
print(str(procnum) + " represent!")
return_dict[procnum] = procnum

if name == "__main__":

manager = multiprocessing.Manager()
return_dict = manager.dict()
jobs = []
for i in range(5):
    p = multiprocessing.Process(target=worker, args=(i, return_dict))
    jobs.append(p)
    p.start()

for proc in jobs:
    proc.join()
print(return_dict.values())

Explanation:

• A multiprocessing Manager is used to create the shared dictionary, 'return_dict'.
• Each worker process accesses the shared 'return_dict' and stores its return value under its process number as the key.
• The main process retrieves the return values by accessing the dictionary's values.

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