I think we are on the same page then, thanks.  

AFAIU, the C-level signal handler results in a flag being set, which is evaluated at some later point in time[1], after a certain number of opcodes have been executed.  Could it be that those opcodes blindly continue to walk into the `else` clause despite the sleep interruption?

[1] https://docs.python.org/3/library/signal.html#execution-of-python-signal-handlers