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up vote 3 down vote favorite

I have code that needs to exit gracefully. To simplify things, I am presenting an example based on the answer given here.

Since I need to handle a few signals, I thought of putting this logic in a function:

def set_signals():
   original_sigint = signal.getsignal(signal.SIGINT)
   signal.signal(signal.SIGINT, exit_gracefully)
   signal.signal(signal.SIGTERM, exit_gracefully)
   signal.signal(signal.SIGINT, exit_gracefully)
   signal.signal(signal.SIGALRM, exit_gracefully)
   signal.signal(signal.SIGHUP, signal.SIG_IGN)

Thus, the main block of the python code should be:

if __name__ == '__main__':
    # store the original SIGINT handler
    set_signals()
    run_program()

This will fail however, since the exit_gracefully does not know the variable original_init.

Therefore, my solution was to create original_sigint as a global variable. 

import signal
import time
import sys


original_sigint = None


def run_program():
    while True:
        time.sleep(1)
        print("a")


def exit_gracefully(signum, frame):
    # restore the original signal handler as otherwise evil things will happen
    # in raw_input when CTRL+C is pressed, and our signal handler is not re-entrant
    global original_sigint
    signal.signal(signal.SIGINT, original_sigint)

    try:
        if raw_input("\nReally quit? (y/n)> ").lower().startswith('y'):
            sys.exit(1)

    except KeyboardInterrupt:
        print("Ok ok, quitting")
        sys.exit(1)

    # restore the exit gracefully handler here
    signal.signal(signal.SIGINT, exit_gracefully)


def set_signals():
    global original_sigint
    original_sigint = signal.getsignal(signal.SIGINT)
    signal.signal(signal.SIGINT, exit_gracefully)
    signal.signal(signal.SIGTERM, exit_gracefully)
    signal.signal(signal.SIGINT, exit_gracefully)
    signal.signal(signal.SIGALRM, exit_gracefully)
    signal.signal(signal.SIGHUP, signal.SIG_IGN)


if __name__ == '__main__':
    # store the original SIGINT handler
    set_signals()
    run_program()

This approach works, but I am not happy with the use of another global variable (the code I working on is quite and is already littered with those). So the question is, how would you do it differently?

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2 Answers

up vote 4 down vote accepted

What about implementing your signal handling code inside a class? This could look something like the following:

class GracefulExit:
    def __enter__(self):
        # set up signals here
        # store old signal handlers as instance variables

    def __exit__(self, type, value, traceback):
        # restore old signal handlers

You can then use this in your code as follows:

with GracefulExit():
    # Signals will be caught inside this block.

# Signals will no more be caught here.

You'll find more examples of how to use the with-statement on the web.

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1  
Hm, this looks much more pythonic :) –  Nobody yesterday
    
See also: contextlib.contextmanager, which allows you to use the with statement above without explicitly implementing __enter__ and __exit__. –  codesparkle yesterday
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up vote 1 down vote

You can avoid the global by passing the original handler as function parameter and binding it with a lambda in set_signals:

def exit_gracefully(signum, frame, original_sigint):
#...

def set_signals():
    original_sigint = signal.getsignal(signal.SIGINT)
    bound_exit_gracefully = lambda signum, frame: exit_gracefully(signum, frame, original_sigint)
    signal.signal(signal.SIGINT, bound_exit_gracefully)
    signal.signal(signal.SIGTERM, bound_exit_gracefully)
    signal.signal(signal.SIGINT, bound_exit_gracefully)
    signal.signal(signal.SIGALRM, bound_exit_gracefully)
    signal.signal(signal.SIGHUP, signal.SIG_IGN)

The naming could also be improved a bit e.g.:

  • set_signals -> setup_grafecul_signal_handler
  • original_sigint -> original_sigint_handler
  • exit_gracefully -> gracefull_exit_signal_handler
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