How to stop python from propagating signals to subprocesses?

Question

I'm using python to manage some simulations. I build the parameters and run the program using:

pipe = open('/dev/null', 'w')
pid = subprocess.Popen(shlex.split(command), stdout=pipe, stderr=pipe)

My code handles different signal. Ctrl+C will stop the simulation, ask if I want to save, and exit gracefully. I have other signal handlers (to force data output for example).

What I want is to send a signal (SIGINT, Ctrl+C) to my python script which will ask the user which signal he wants to send to the program.

The only thing preventing the code to work is that it seems that whatever I do, Ctrl+C will be "forwarded" to the subprocess: the code will catch it to and exit:

try:
  <wait for available slots>
except KeyboardInterrupt:
  print "KeyboardInterrupt catched! All simulations are paused. Please choose the signal to send:"
  print "  0: SIGCONT (Continue simulation)"
  print "  1: SIGINT  (Exit and save)"
  [...]
  answer = raw_input()
  pid.send_signal(signal.SIGCONT)
  if   (answer == "0"):
    print "    --> Continuing simulation..."
  elif (answer == "1"):
    print "    --> Exit and save."
    pid.send_signal(signal.SIGINT)
    [...]

So whatever I do, the program is receiving the SIGINT that I only want my python script to see. How can I do that???

I also tried:

signal.signal(signal.SIGINT, signal.SIG_IGN)
pid = subprocess.Popen(shlex.split(command), stdout=pipe, stderr=pipe)
signal.signal(signal.SIGINT, signal.SIG_DFL)

to run the program but this gives the same result: the program catches the SIGINT.

Thanx!

Answer 1

Combining some of other answers that will do the trick - no signal sent to main app will be forwarded to the subprocess.

import os
from subprocess import Popen

def preexec(): # Don't forward signals.
    os.setpgrp()

Popen('whatever', preexec_fn = preexec)