Adam, forbidding signals + threads is not possible.  signalmodule is _always_ imported by python; it is builtin.  That's because it is signal module that provides support for raising KeyboardInterrupt when Ctrl-C is pressed.  Therefore. "signals + threads == forbidden" simplifies to "threads == forbidden", and if threads are forbidden then let's just say Python loses a lot of support :)

Regarding async safety, I think that generally any system call that may potentially suspend a process, such as blocking system calls and, most notably, mutexes, are not safe to use in async handlers.  Writing to a pipe is safe only because the file descriptor is in non-blocking mode.  OTOH, in non-blocking mode the file descriptor can only accept a few bytes at a time, hence the (documented) limitation of not accepting large bursts of signals.