Python 3.8 support

I know that Django doesn’t officially support Python 3.8 yet, but I tried an upgrade anyway. Django seems to work fine and our project is happy, but qcluster crashes on start with:

TypeError: cannot pickle '_thread.lock' object

Seems to be related to multiprocessing changes in py38. As far as I can tell at this early stage, it only seems to crash on MacOS, while Linux is fine (our Linux developers had no problem and our server deploys were fine, but MacOS devs all experienced this crash).

Discussion of this started in #313, but this is a separate issue.

[roharvey]

As @Solanar said, this is related to the start method. A very simple workaround is to include this in startup (windows or OS X, but should be a no-op in linux):

from multiprocessing import set_start_method
set_start_method("fork")

The reason this shows up in python3.8 is because the default multiprocessing method changed: https://docs.python.org/3/whatsnew/3.8.html#multiprocessing

[jeroenbrouwer]

As @Solanar said, this is related to the start method. A very simple workaround is to include this in startup (windows or OS X, but should be a no-op in linux):

from multiprocessing import set_start_method
set_start_method("fork")

The reason this shows up in python3.8 is because the default multiprocessing method changed: https://docs.python.org/3/whatsnew/3.8.html#multiprocessing

Thanks for the suggestion! I’ve added the following snippet to my manage.py and it works 😃

import platform
if platform.system() != "Linux":
    from multiprocessing import set_start_method
    set_start_method("fork")