Why does Python's multiprocessing module import __main__ when starting a new process on Windows?

Question

I am playing around with a library for my beginner students, and I'm using the multiprocessing module in Python. I ran into this problem: importing and using a module that uses multiprocessing without causing infinite loop on Windows

As an example, suppose I have a module mylibrary.py :

# mylibrary.py

from multiprocessing import Process

class MyProcess(Process):
    def run(self):
        print "Hello from the new process"

def foo():
    p = MyProcess()
    p.start()

And a main program that calls this library:

# main.py

import mylibrary

mylibrary.foo()

If I run main.py on Windows, it tries to import main.py into the new process, meaning the code is executed again which results in an infinite loop of process generation. I can fix it like so:

import mylibrary

if __name__ == "__main__":
    mylibrary.foo()

But, this is pretty confusing for beginners, and moreover it seems like it shouldn't be necessary. The new process is being created in mylibrary , so why doesn't the new process just import mylibrary ? Is there a way to work around this issue without having to change main.py ?

I am using Python 2.7, by the way.

Answer 1

Windows doesn't have fork , so there's no way to make a new process just like the existing one. So the child process has to run your code again, but now you need a way to distinguish between the parent process and the child process, and __main__ is it.

This is covered in the docs here: http://docs.python.org/2/library/multiprocessing.html#windows

I don't know of another way to structure the code to avoid the fork bomb effect.