[TIP] Mock and multiprocessing on Windows

[akira]

On 09/25/2012 12:23 PM, Stefan Scherfke wrote:
> 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
the fork semantics allows the child process to see changes to globals 
introduced in the parent on Mac,Linux. mock.patch should be executed in 
each process on Windows.