controls – How do I capture SIGINT in Python?

The Question :

568 people think this question is useful

I’m working on a python script that starts several processes and database connections. Every now and then I want to kill the script with a Ctrl+C signal, and I’d like to do some cleanup.

In Perl I’d do this:

$SIG{'INT'} = 'exit_gracefully';

sub exit_gracefully {
    print "Caught ^C \n";
    exit (0);
}

How do I do the analogue of this in Python?

The Question Comments :

The Answer 1

838 people think this answer is useful

Register your handler with signal.signal like this:

#!/usr/bin/env python
import signal
import sys

def signal_handler(sig, frame):
    print('You pressed Ctrl+C!')
    sys.exit(0)
signal.signal(signal.SIGINT, signal_handler)
print('Press Ctrl+C')
signal.pause()

Code adapted from here.

More documentation on signal can be found here.  

The Answer 2

182 people think this answer is useful

You can treat it like an exception (KeyboardInterrupt), like any other. Make a new file and run it from your shell with the following contents to see what I mean:

import time, sys

x = 1
while True:
    try:
        print x
        time.sleep(.3)
        x += 1
    except KeyboardInterrupt:
        print "Bye"
        sys.exit()

The Answer 3

70 people think this answer is useful

And as a context manager:

import signal

class GracefulInterruptHandler(object):

    def __init__(self, sig=signal.SIGINT):
        self.sig = sig

    def __enter__(self):

        self.interrupted = False
        self.released = False

        self.original_handler = signal.getsignal(self.sig)

        def handler(signum, frame):
            self.release()
            self.interrupted = True

        signal.signal(self.sig, handler)

        return self

    def __exit__(self, type, value, tb):
        self.release()

    def release(self):

        if self.released:
            return False

        signal.signal(self.sig, self.original_handler)

        self.released = True

        return True

To use:

with GracefulInterruptHandler() as h:
    for i in xrange(1000):
        print "..."
        time.sleep(1)
        if h.interrupted:
            print "interrupted!"
            time.sleep(2)
            break

Nested handlers:

with GracefulInterruptHandler() as h1:
    while True:
        print "(1)..."
        time.sleep(1)
        with GracefulInterruptHandler() as h2:
            while True:
                print "\t(2)..."
                time.sleep(1)
                if h2.interrupted:
                    print "\t(2) interrupted!"
                    time.sleep(2)
                    break
        if h1.interrupted:
            print "(1) interrupted!"
            time.sleep(2)
            break

From here: https://gist.github.com/2907502

The Answer 4

29 people think this answer is useful

You can handle CTRL+C by catching the KeyboardInterrupt exception. You can implement any clean-up code in the exception handler.

The Answer 5

23 people think this answer is useful

Yet Another Snippet

Referred main as the main function and exit_gracefully as the CTRL + c handler

if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        pass
    finally:
        exit_gracefully()

The Answer 6

22 people think this answer is useful

From Python’s documentation:

import signal
import time

def handler(signum, frame):
    print 'Here you go'

signal.signal(signal.SIGINT, handler)

time.sleep(10) # Press Ctrl+c here

The Answer 7

10 people think this answer is useful

I adapted the code from @udi to support multiple signals (nothing fancy) :

class GracefulInterruptHandler(object):
    def __init__(self, signals=(signal.SIGINT, signal.SIGTERM)):
        self.signals = signals
        self.original_handlers = {}

    def __enter__(self):
        self.interrupted = False
        self.released = False

        for sig in self.signals:
            self.original_handlers[sig] = signal.getsignal(sig)
            signal.signal(sig, self.handler)

        return self

    def handler(self, signum, frame):
        self.release()
        self.interrupted = True

    def __exit__(self, type, value, tb):
        self.release()

    def release(self):
        if self.released:
            return False

        for sig in self.signals:
            signal.signal(sig, self.original_handlers[sig])

        self.released = True
        return True

This code support the keyboard interrupt call (SIGINT) and the SIGTERM (kill <process>)

The Answer 8

8 people think this answer is useful

In contrast to Matt J his answer, I use a simple object. This gives me the possibily to parse this handler to all the threads that needs to be stopped securlery.

class SIGINT_handler():
    def __init__(self):
        self.SIGINT = False

    def signal_handler(self, signal, frame):
        print('You pressed Ctrl+C!')
        self.SIGINT = True


handler = SIGINT_handler()
signal.signal(signal.SIGINT, handler.signal_handler)

Elsewhere

while True:
    # task
    if handler.SIGINT:
        break

The Answer 9

5 people think this answer is useful

If you want to ensure that your cleanup process finishes I would add on to Matt J’s answer by using a SIG_IGN so that further SIGINT are ignored which will prevent your cleanup from being interrupted.

import signal
import sys

def signal_handler(signum, frame):
    signal.signal(signum, signal.SIG_IGN) # ignore additional signals
    cleanup() # give your process a chance to clean up
    sys.exit(0)

signal.signal(signal.SIGINT, signal_handler) # register the signal with the signal handler first
do_stuff()

The Answer 10

4 people think this answer is useful

You can use the functions in Python’s built-in signal module to set up signal handlers in python. Specifically the signal.signal(signalnum, handler) function is used to register the handler function for signal signalnum.

The Answer 11

3 people think this answer is useful

thanks for existing answers, but added signal.getsignal()

import signal

# store default handler of signal.SIGINT
default_handler = signal.getsignal(signal.SIGINT)
catch_count = 0

def handler(signum, frame):
    global default_handler, catch_count
    catch_count += 1
    print ('wait:', catch_count)
    if catch_count > 3:
        # recover handler for signal.SIGINT
        signal.signal(signal.SIGINT, default_handler)
        print('expecting KeyboardInterrupt')

signal.signal(signal.SIGINT, handler)
print('Press Ctrl+c here')

while True:
    pass

The Answer 12

0 people think this answer is useful

Personally, I couldn’t use try/except KeyboardInterrupt because I was using standard socket (IPC) mode which is blocking. So the SIGINT was cueued, but came only after receiving data on the socket.

Setting a signal handler behaves the same.

On the other hand, this only works for an actual terminal. Other starting environments might not accept Ctrl+C, or pre-handle the signal.

Also, there are “Exceptions” and “BaseExceptions” in Python, which differ in the sense that interpreter needs to exit cleanly itself, so some exceptions have a higher priority than others (Exceptions is derived from BaseException)

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