Thanks for the detailed analysis, Charles-François.

> Finally, I think that the documentation should be rephrased:

Yes, I think so.

> Furthermore, under Linux 2.6 and NPTL, getpid() returns the main thread
> PID even from another thread.

Yes, those threads belong to the same process.

But as mentioned, signals are a rather fragile inter-process communication device; just use a specific file descriptor.
And if you still wanna use signals, there's set_wakeup_fd():
http://docs.python.org/library/signal.html#signal.set_wakeup_fd