Fork error on MacOS

[garysb]

When running in standalone mode on MacOS, having a window open another window causes the following error:

The process has forked and you cannot use this CoreFoundation functionality safely. You MUST exec().
Break on __THE_PROCESS_HAS_FORKED_AND_YOU_CANNOT_USE_THIS_COREFOUNDATION_FUNCTIONALITY___YOU_MUST_EXEC__() to debug.

This seems to be related to the _MultiAppLaunch class within standalone.py

[huntfx]

I googled the error and it seems to be a bug with the multiprocessing module.

I'm not sure if this line is Python 3 specific, but this apparently solves it if you were to add it to the top of the file:

if platform.system() == 'Darwin':
    multiprocessing.set_start_method('spawn')

There's also a workaround someone came up with in case you're using Python 2, I can't tell if it'll be of any help though.

If you were to use win.show(instance=True), then it will not spawn a new process, but as a downside certain things will be shared between the windows such as the palette. If you're working in Python 3 and the top solution works for you, I'll just default this to True for MacOS Py2 to avoid the error.