[Python-ideas] Context manager to temporarily set signal handlers

[Thomas Kluyver]

Not uncommonly, I want to do something like this in code:

import signal

# Install my own signal handler
prev_hup = signal.signal(signal.SIGHUP, my_handler)
prev_term = signal.signal(signal.SIGTERM, my_handler)
try:
    do_something_else()
finally:
    # Restore previous signal handlers
    signal.signal(signal.SIGHUP, prev_hup)
    signal.signal(signal.SIGTERM, prev_term)

This works if the existing signal handler is a Python function, or the
special values SIG_IGN (ignore) or SIG_DFL (default). However, it breaks
if code has set a signal handler in C: this is not returned, and there
is no way in Python to reinstate a C-level signal handler once we've
replaced it from Python.

I propose two possible solutions:

1. The high-level approach: a context manager which can temporarily set
one or more signal handlers. If this was implemented in C, it could
restore C-level as well as Python-level signal handlers.

2. A lower level approach: signal() and getsignal() would gain the
ability to return an opaque object which refers to a C-level signal
handler. The only use for this would be to pass it back to
signal.signal() to set it as a signal handler again. The context manager
from (1) could then be implemented in Python.

Crosslinking http://bugs.python.org/issue13285

Thomas