Many of the differences between threading and multiprocessing are not really Python-specific, and some differences are specific to a certain Python implementation.
For CPython, I would use the multiprocessing module in either fo the following cases:
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I need to make use of multiple cores simultaneously for performance reasons. The global interpreter lock (GIL) would prevent any speedup when using threads. (Sometimes you can get away with threads in this case anyway, for example when the main work is done in C code called via
ctypesor when using Cython and explicitly releasing the GIL where approriate. Of course the latter requires extra care.) Note that this case is actually rather rare. Most applications are not limited by processor time, and if they really are, you usually don’t use Python. -
I want to turn my application into a real distributed application later. This is a lot easier to do for a multiprocessing application.
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There is very little shared state needed between the the tasks to be performed.
In almost all other circumstances, I would use threads. (This includes making GUI applications responsive.)