multithreading - When are Python threads fast?

Question

We're all aware of the horrors of the GIL, and I've seen a lot of discussion about the right time to use the multiprocessing module, but I still don't feel that I have a good intuition about when threading in Python (focusing mainly on CPython) is the right answer.

What are instances in which the GIL is not a significant bottleneck? What are the types of use cases where threading is the most appropriate answer?

Answer 1

Threading really only makes sense if you have a lot of blocking I/O going on. If that's the case, then some threads can sleep while other threads work. If threads are CPU-bound, you're not likely to see much benefit from multithreading.

Note that the multiprocessing module, while more difficult to code for, makes use of separate processes and therefore doesn't suffer the downsides of the GIL.