python - Multiprocessing Bomb

Question

I was working the following example from Doug Hellmann tutorial on multiprocessing:

import multiprocessing

def worker():
    """worker function"""
    print 'Worker'
    return

if __name__ == '__main__':
    jobs = []
    for i in range(5):
        p = multiprocessing.Process(target=worker)
        jobs.append(p)
        p.start()

When I tried to run it outside the if statement:

import multiprocessing

def worker():
    """worker function"""
    print 'Worker'
    return

jobs = []
for i in range(5):
    p = multiprocessing.Process(target=worker)
    jobs.append(p)
    p.start()

It started spawning processes non-stop, and the only way to stop it was reboot!

Why would that happen? Why it did not generate 5 processes and exit? Why do I need the if statement?

Answer 1

On Windows there is no fork() routine, so multiprocessing imports the current module to get access to the worker function. Without the if statement the child process starts its own children and so on.