Python multiprocessing, share data between process using an object

Question

I'm trying to use a generic object called 'Memory' to temporary store a value that is writen by a process called 'Writer_process' and readed by 'Reader_process' but reader return only the original value stored in 'Memory'.

import multiprocessing
import time

class Writer_process(multiprocessing.Process):
    def __init__(self, val, memory):
        super().__init__()
        self.val = val
        self.memory = memory

    def run(self):
        cont = 0
        while True:
            cont += self.val
            self.memory.num = cont
            time.sleep(1)

class Reader_process(multiprocessing.Process):
    def __init__(self, val, memory):
        super().__init__()
        self.val = val
        self.memory = memory

    def run(self):
        while True:
            print(self.memory.num)
            time.sleep(self.val)

class Memory():
    num = 0

if __name__ == '__main__':
    memory = Memory()
    writer = Writer_process(1, memory)
    reader = Reader_process(1, memory)
    writer.start()
    reader.start()

If you need real shared mutable state in multiprocessing, you will have to use low-level primitives. It is really best to avoid shared mutable state in multiprocessing, though. By the way, judging by the names of your objects, what you want is an IO-bound reader/writer-pair. In that case it's best to use asynchronous execution instead of multiprocessing. — Eli Korvigo
– Eli Korvigo, Commented Sep 27, 2018 at 23:31

Fabio Cescon · Accepted Answer · 2018-09-28 09:27:47Z

1

Thank you to Eli Korvigo I solved the problem using a 'Manager' to manage the access to the memory and a specific multiprocessing object called 'Value' can allow me to store a single data and share it betwneen different processes.

import multiprocessing
import time

class Writer_process(multiprocessing.Process):
    def __init__(self, val, memory):
        super().__init__()
        self.val = val
        self.memory = memory

    def run(self):
        cont = 0
        while True:
            cont += 1
            self.memory.value = cont
            time.sleep(self.val)

class Reader_process(multiprocessing.Process):
    def __init__(self, val, memory):
        super().__init__()
        self.val = val
        self.memory = memory

    def run(self):
        while True:
            self.datum = self.memory.value
            print(self.datum)
            time.sleep(self.val)

if __name__ == '__main__':
    with multiprocessing.Manager() as manager:
        memory = manager.Value('i', 0)
        writer = Writer_process(1, memory)
        reader = Reader_process(1, memory)
        writer.start()
        reader.start()
        writer.join()
        reader.join()

answered Sep 28, 2018 at 9:27

Fabio Cescon

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1 Comment

Eli Korvigo Over a year ago

I'm glad you've solved your issue. You can now accept your own answer.

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