What are the Python thread + Unix signals semantics?

Question

What are the rules surrounding Python threads and how Unix signals are handled?

Is KeyboardInterrupt, which is triggered by SIGINT but handled internally by the Python runtime, handled differently?

Answer 1

First, when setting up signal handlers using the signal module, you must create them in the main thread. You will receive an exception if you try to create them in a separate thread.

Signal handlers registered via the signal.signal() function will always be called in the main thread. On architectures which support sending signals to threads, at the C level I believe the Python runtime ignores all signals on threads and has a signal handler on the main thread, which it uses to dispatch to your Python-code signal handler.

The documentation for the thread module states that the KeyboardInterrupt exception (which is ordinarily triggered by SIGINT) can be delivered to an arbitrary thread unless you have the signal module available to you, which all Unix systems should have. In that case, it's delivered to the main thread. If you're on a system without signal, you'll have to catch KeyboardInterrupt in your thread and call thread.interrupt_main() to re-raise it in the main thread.

More information can be found in the Python docs for the thread and signal modules.