Expected python bug using multiprocessing and inheritance #96469

bsdis · 2022-09-01T00:53:19Z

This is a bug report:

import time
import multiprocessing as mp
class A(object):
    def __init__(self, childnodes, **kwargs) -> None:
        self._childnodes = childnodes
        self._kwargs = kwargs

    def process(self, datablock, meta):
        print("Hello")
        time.sleep(1)
        print("world")

class B(A):
    def process(self, datablock, meta):
        print("before")
        mp.Process(target=super().process, args=(self, datablock, meta)).start()
        print("after")

if __name__ == "__main__":
    b = B([1,2,3])
    b.process([4,5,6], {'foo':42})

This seems to run forerever which is not expected. Reason seems to be that that somehow, super().process actually calls B.process instead of A.process in the subprocess. This is not expected behaviour. As additional evidence towards that, the program behaviour can be corrected by changing

mp.Process(target=super().process, args=(self, datablock, meta)).start()`meta)).start()

with

mp.Process(target=A.process, args=(self, datablock, meta)).start()

Machine:
MacBook Pro (16-inch, 2021)
MacOS Monteray 12.5
Chip: Apple M1 Max
Python version: 3.9.12

The text was updated successfully, but these errors were encountered:

saito828koki · 2022-09-05T02:19:21Z

@bsdis

Hello,
Thanks for your interesting bug report!

To summarize that unexpected behavior, super().process method is somehow overridden when calling
multiprocessing.Process.start().
I found that we can get the expected result by using multiprocessing.Process.run().

import multiprocessing as mp


class A(object):
    def process(self):
        print("method of A")


class B(A):
    def foo(self):
        mp.Process(target=super().process).run()  # "method of A" is printed
        mp.Process(target=super().process).start()  # "method of B" is printed

    def process(self):
        print("method of B")


if __name__ == "__main__":
    b = B()
    b.foo()

I will dive deeper to find what makes that difference.

bsdis added the type-bug An unexpected behavior, bug, or error label Sep 1, 2022

Expected python bug using multiprocessing and inheritance #96469

Expected python bug using multiprocessing and inheritance #96469

bsdis commented Sep 1, 2022 •

edited

saito828koki commented Sep 5, 2022

Expected python bug using multiprocessing and inheritance #96469

Expected python bug using multiprocessing and inheritance #96469

Comments

bsdis commented Sep 1, 2022 • edited

saito828koki commented Sep 5, 2022

bsdis commented Sep 1, 2022 •

edited