Multiprocessing by default uses the fork system call to start new processes on Linux. This system call is not available on Windows, and there multiprocessing starts a fresh interpreter (see <https://docs.python.org/3.6/library/multiprocessing.html#multiprocessing.get_start_method>). 

I'm also on macOS, and cannot reproduce the problem there even when using the 'spawn' method there by adding some lines to the start of your script (before the other import statements):

import multiprocessing
if __name__ == "__main__":
    multiprocessing.set_start_method('spawn')

But: I have a fairly old version of 3.6 on my machine.