There is an argument that if you need in process multithreading you should use a different language. But a lot of people need to use python because everything else they’re doing is in python.

There are quite a few common cases where in process multi threading is useful. The main ones are where you have large inputs or large outputs to the work units. In process is nice because you can move the input or output state to the work units instead of having to copy it.

One very common case is almost all gui applications. Where you want to be able to do all work on background threads and just move data back and forth from the coordinating ui thread. JavaScript’s lack of support here, outside of a native language compiled into emscripten, is one reason web apps are so hard to make jankless. The copies of data across web workers or python processes are quite expensive as far as things go.

Once a week or so, I run into a high compute python scenario where the existing forms of multiprocessing fail me. Large shared inputs and or don’t want the multiprocess overhead; but GIL slows everything down.