My problem was actually related to the subprocess.Popen I use inside my threads in combination with signals. As Popen does not spawn a new thread, but a new process, it completely ignores my locks.

However, it seems impossible to safely unregister signals for subprocesses. I tried using SIG_IGN in preexec_fn for Popen as well as a ignore/Popen/unignore wrapper, but both result in race conditions (for the first: if the signal arrives between fork and exec and for the second if it arrives within the wrapper).

I think ignoring signals by default for everything but the main thread or process would solve this problem and make parallel programming with events in Python a lot easier.

Explicit is better than implicit.