multiprocessing breaks in interactive mode

Question

I have the following code

from multiprocessing import Process, Queue
from queue import Empty
from time import sleep

def f(q):
    n = 100000000
    while n != 100000000 // 2:
        n -= 1
    q.put("the awkening!")
    print("my work here is done")

def main():
    q = Queue()
    p = Process(target=f, args=(q,))
    p.start()
    while True:
        try:
            print(q.get(block=False))
            raise systemexit
        except Empty:
            print("i found nothing :(")
            sleep(2)
    p.join()

If I add

if __name__ == '__main__':
     main()

To the end then use python script_name.py to run it, everything works fine. However, if I just run the scirpt using python -i script_name.py then run main() Python complains:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Python34\lib\multiprocessing\spawn.py", line 98, in spawn_main
    exitcode = _main(fd)
  File "C:\Python34\lib\multiprocessing\spawn.py", line 108, in _main
    self = pickle.load(from_parent)
AttributeError: Can't get attribute 'f' on <module '__main__' (built-in)>

The error comes from the child process, the main process runs fine.

This is not a big deal, but I wonder why this happens, also it would be nice if it works in interactive mode

Answer 1

The multiprocessing documentation discusses this:

Note

Functionality within this package requires that the __main__ module be importable by the children. This is covered in Programming guidelines however it is worth pointing out here. This means that some examples, such as the multiprocessing.Pool examples will not work in the interactive interpreter.

My understanding is that __main__ is defined very differently in the context of an interactive session (as it is associated with the shell, not the file that is running).