shared.py

import multiprocessing

lock = multiprocessing.Lock()

class Process(multiprocessing.Process):
    
    def __init__(self, counter):
        super(Process, self).__init__()
        self.counter = counter

    def run(self):
        for i in range(1000):
            with lock:
                self.counter.value += 1


def main():
    counter = multiprocessing.Value('i', lock=True)
    counter.value = 0
    
    processes = [Process(counter) for i in range(4)]
    [p.start() for p in processes]
    [p.join() for p in processes]
    print(counter.value)


if __name__ == '__main__':
    main()