ProcessPoolExecutor child process problem in windows

[pullgon]

Bug report

Bug description:

Operating system：Win10

import os
from concurrent.futures import ProcessPoolExecutor, ThreadPoolExecutor

print(f"001 at {os.getpid()}")


def task():
    print("hello world")


# print("002")
if __name__ == '__main__':
    print(f"002 --- {os.getpid()}")
    pool = ProcessPoolExecutor(max_workers=10)
    pool.submit(task, *())
    pool.shutdown(True)

Unexpected output，Why does 001 appear twice?

001 at 18836
002 --- 18836
001 at 68124
hello world

I tried it on linux without any problems，I guess ProcessPoolExecutor is not so friendly for windows support.

CPython versions tested on:

3.8, 3.10, 3.11

Operating systems tested on:

Windows

[kirill-ratkin]

Hi. This is well known behavior on Windows because Windows spawns processes, not forks it.
You could see this article: https://www.pythonforthelab.com/blog/differences-between-multiprocessing-windows-and-linux/

[rockpradeep143]

Linux uses forking to create new processes and it copies the parent memory so the child process has the states and variables of the parent process and not reload the module file again. the code outside the main thread is executed only once.
Window restarts the python interpretter when a new process created, so to prevent the file to run again python uses this construct if name == 'main':
The if name == 'main': construct ensures that the code inside this block only runs if the script is being run as the main program. Since the child process doesn't have name == 'main'