[TIP] Mock and multiprocessing on Windows

[Stefan Scherfke]

Hi,

[tl;dr] Below is a snippet that works on Linux/Mac but fails on Windows. Is there a way to fix it?

The example below fails on Windows, because it uses spawn() to start subprocesses. The python
state is being pickled and sent to the new process. However, pickling mock objects doesn’t
work, so the example fails.

Is there another way to mock something in a subprocess on Windows?


Below is a minimal example that prints 

    <class 'mock.MagicMock'>
   eggs

on Mac/Linux and:

   <class 'function'>
   spam

on Windows:


# sub.py
def spam():
   return 'spam'


def main():
   print(type(spam))
   print(spam())

# EOF

# main.py
import multiprocessing
import mock

import sub


def main():
   with mock.patch('sub.spam') as spam_mock:
       spam_mock.return_value = 'eggs'
       p = multiprocessing.Process(target=sub.main)
       p.start()
       p.join()


if __name__ == '__main__':
   main()

# EOF

Cheers,
Stefan
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