Question

I have a module named multi.py. If I simply wanted to execute multi.py as a script, then the workaround to avoid crashing on Windows (spawning an infinite number of processes) is to put the multiprocessing code under:

if __name__ == '__main__':

However, I am trying to import it as a module from another script and call multi.start(). How can this be accomplished?

# multi.py
import multiprocessing

def test(x):
    x**=2

def start():
    pool = multiprocessing.Pool(processes=multiprocessing.cpu_count()-2)
    pool.map(test, (i for i in range(1000*1000)))
    pool.terminate()
    print('done.')

if __name__ == '__main__':
    print('runs as a script,',__name__)
else:
    print('runs as imported module,',__name__)

This is my test.py I run:

# test.py
import multi
multi.start()

Answer 1

I don’t quite get what you’re asking. You don’t need to do anything to prevent this from spawning infinitely many processes. I just ran it on Windows XP — imported the file and ran multi.start() — and it completed fine in a couple seconds.

The reason you have to do the if __name__=="__main__" protection is that, on Windows, multiprocessing has to import the main script in order to run the target function, which means top-level module code in that file will be executed. The problem only arises if that top-level module code itself tries to spawn a new process. In your example, the top level module code doesn’t use multiprocessing, so there’s no infinite process chain.

Edit: Now I get what you’re asking. You don’t need to protect multi.py. You need to protect your main script, whatever it is. If you’re getting a crash, it’s because in your main script you are doing multi.start() in the top level module code. Your script needs to look like this:

import multi
if __name__=="__main__":
    multi.start()

The “protection” is always needed in the main script.