How to stop SIGINT being passed to subprocess in python?

Question

My python script intercepts the SIGINT signal with the signal process module to prevent premature exit, but this signal is passed to a subprocess that I open with Popen. is there some way to prevent passing this signal to the subprocess so that it also is not exited prematurely when the user presses ctrl-c?

Answer 1

Signal handlers are inherited when you start a subprocess, so if you use the signal module to ignore SIGINT (signal.signal(signal.SIGINT, signal.SIG_IGN)), then your child process automatically will also.

There are two important caveats, though:

• You have to set the ignore handler before you spawn the child process
• Custom signal handlers are reset to the default handlers, since the child process won't have access to the handler code to run it.

So if you need to customise your handling of SIGINT rather than just ignoring it, you probably want to temporarily ignore SIGINT while you spawn your child process, then (re)set your custom signal handler.

If you're trying to catch SIGINT and set a flag so you can exit at a safe point rather than immediately, remember that when you get to that safe point your code will have to manually clean up its descendants, since your child process and any processes it starts will be ignoring the SIGINT.

Answer 2

You are able to re-assign the role of ctrl-c using the tty module, which allows you to manipulate the assignment of signals. Be warned, however, that unless you put them back the way they were before you modified them, they will persist for the shell's entire session, even after the program exits.

Here is a simple code snippet to get you started that stores your old tty settings, re-assigns ctrl-c to ctrl-x, and then restores your previous tty settings upon exit.

import sys
import tty

# Back up previous tty settings
stdin_fileno = sys.stdin.fileno()
old_ttyattr = tty.tcgetattr(stdin_fileno)

try:
    print 'Reassigning ctrl-c to ctrl-x'

    # Enter raw mode on local tty
    tty.setraw(stdin_fileno)
    raw_ta = tty.tcgetattr(stdin_fileno)
    raw_ta[tty.LFLAG] |= tty.ISIG
    raw_ta[tty.OFLAG] |= tty.OPOST | tty.ONLCR

    # ^X is the new ^C, set this to 0 to disable it entirely
    raw_ta[tty.CC][tty.VINTR] = '\x18'  

    # Set raw tty as active tty
    tty.tcsetattr(stdin_fileno, tty.TCSANOW, raw_ta)

    # Dummy program loop
    import time
    for _ in range(5):
        print 'doing stuff'
        time.sleep(1)

finally:
    print 'Resetting ctrl-c'
    # Restore previous tty no matter what
    tty.tcsetattr(stdin_fileno, tty.TCSANOW, old_ttyattr)