Inject a signal handler before the existed signal handler.

example.py

import signal
import time
from signal_inject import *

def dummy_handler(signum, frame):
    print "In dummy handler"

inject_signal_handler(signal.SIGINT, dummy_handler)

while 1:
    time.sleep(1)

signal_inject.py

import signal


def inject_signal_handler(signum, handler):
    '''
    Execute the handler before the existed handler.
    '''
    # Get the old handler
    old_handler = signal.getsignal(signum)
    # Create a new combined handler
    def new_handler(*sub):
        handler(*sub)
        if callable(old_handler):
            old_handler(*sub)
    # Re-bind the handler
    signal.signal(signum, new_handler)