Default signal handling in Python

Question

I would like to register a custom signal handler that does some things and then continues with the normal signal handling, specifically for the SIGTERM signal.

Python does not have a SIGTERM handler registered ( Python: What is the default handling of SIGTERM? ), so how can I continue with the OS's normal SIGTERM processing after I do my own custom handling?

def my_handler(self, signum, frame):
    do_something()
    # What do I do here to raise to the OS?

signal.signal(signal.SIGTERM, my_handler)

Answer 1

If the fatal signal's previous disposition was the default, the best you can do is restore that disposition and re-raise the signal.

This will at least cause your process to terminate abnormally, such that its parent can tell which signal killed it ( os.WIFSIGNALED ). If the fatal signal has an "additional action" like a core file, this mimicry will not be perfect.

orig_handler = signal.signal(signal.SIGTERM, my_handler)

def my_handler(signum, frame):
  do_something()

  # Now do whatever we'd have done if
  # my_handler had not been installed:

  if callable(orig_handler):              # Call previous handler
    orig_handler(signum, frame)
  elif orig_handler == signal.SIG_DFL:    # Default disposition
    signal.signal(signum, signal.SIG_DFL)
    os.kill(os.getpid(), signum)
                                          # else SIG_IGN - do nothing

Re-raising the signal as above isn't nuanced enough for signals that stop the process (eg, SIGTSTP) or are ignored by default (SIGCHLD, SIGURG).

Answer 2

You want to save the default one before, as this:

def my_handler(self, signum, frame):
    global original_sig_handler # Doesn't really matter if you do it as global or something else, you just need  the function to get here
    do_something()
    original_sig_handler()

original_sig_handler = signal.getsignal(signal.SIGINT) # this return the function that called by handling the signal
signal.signal(signal.SIGINT, my_handler)

EDIT:

this will not work with sigterm but will work for few other signals.

probably for sigterm python do something as sys.exit or ect.