How to Share and Modify a Dictionary Between Multiple Processes in Python?

Multiprocessing: Inter-Process Dictionary Sharing

Consider a scenario where multiple processes operate on a shared queue, Q, and manipulate a global dictionary, D. Surprisingly, accessing D after joining Q reveals an empty dictionary, despite observed modifications in child processes.

This behavior stems from synchronization issues. Each process operates on its own memory space, and changes made to D within child processes are not automatically reflected in the main process. To address this, synchronization measures are necessary.

Solution: Using a Manager Object

The Python standard library provides a solution using the Manager object. This object offers synchronized access to shared data among processes:

from multiprocessing import Process, Manager

def f(d):
    d[1] += '1'
    d['2'] += 2

if __name__ == '__main__':
    manager = Manager()

    d = manager.dict()
    d[1] = '1'
    d['2'] = 2

    p1 = Process(target=f, args=(d,))
    p2 = Process(target=f, args=(d,))
    p1.start()
    p2.start()
    p1.join()
    p2.join()

    print(d)

In this example, the Manager object's dict() method creates a shared dictionary, d, which is accessible by all processes. Changes made to d within the child processes are propagated to the main process through the shared memory managed by the Manager object.

Output:

$ python mul.py
{1: '111', '2': 6}

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