I think that the problem here is that the documentation sets an expectation that's not met, at least from my POV. Running the reproducer, I was expecting the main thread to unpause and execute the signal handler (citing: "Python signal handlers are always executed in the main Python thread"). I understand that this may be insanely hard to fix in code, so documentation should explicitly point out that there are some (perhaps unfixable) problems with Python's approach to threading+signals.