In this case that's before the next bytecode executes. The write syscall gets interrupted and returns EINTR, then cpython checks what signal was caught and executes the signal handler, before trying to do the remaining write:

https://github.com/python/cpython/blob/5f7aba938cf5007b6f954...

Here's even a test for it: https://github.com/python/cpython/blob/5f7aba938cf5007b6f954...