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.