How to Retrieve Return Values from Multiprocessing.Process Functions in Python?

Access Return Value of a Function in Multiprocessing.Process

In multiprocessing with Python, obtaining the return value of a function passed to multiprocessing.Process can be tricky, as it is not stored directly in the Process object.

Consider the following example:

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)

While this code prints the output of each worker process, we cannot directly access their return values.

To retrieve the return values, we can employ a shared variable. In the following revised code, a Manager() from the multiprocessing module is used to create a shared dictionary:

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())

Here, each worker process updates the shared dictionary return_dict with its return value. The main process can then access these values from the shared dictionary.

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