Is there a way to never exit on KeyboardInterrupt in python?

Question

I'm creating sort of a interactive command line in python. I have something like this:

def complete_menu():
    while True:
        cmd = input('cmd> ')
        if cmd == "help":
            print('help')
        elif cmd == "?":
            print('?')

When the user presses CTRL-C, instead of exiting the program I'm trying to make it so that it prints "please type exit to exit" and goes back to the while True. I have something like this at the moment:

if __name__ == "__main__":
    try:
       main()
    except KeyboardInterrupt:
        print('Please use exit to exit')
        complete_menu()

Although this works, there is a number of issues. For one, when CTRL-C is pressed the first time, it prints out the text and works perfectly. However, the second time the user presses CTRL-C, it exists with a bunch of messy text like any other program after pressing CTRL-C. Can this be fixed?

wim · Accepted Answer · 2019-11-23 00:38:35Z

4

The better way to do this is to register a signal handler:

import signal

def handler(signum, frame):
    print("Please use exit to exit")
    # or: just call sys.exit("goodbye")

...

def main():
    signal.signal(signal.SIGINT, handler)  # prevent "crashing" with ctrl+C
    ...


if __name__ == "__main__":
    main()

Now when a Ctrl+C is received in your code, instead of a KeyboardInterrupt exception being raised, the function handler will be executed. This is a basic example, customize the code within handler to do what you want.

Note: My recommendation is to actually let the user exit with Ctrl+C, i.e. execute any cleanup code that you might need to run and then call sys.exit here. Programs that require a stronger signal to kill are annoying.

edited Nov 23, 2019 at 0:38

answered Nov 23, 2019 at 0:32

wim

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3 Comments

Peter Badida Over a year ago

Much cleaner solution than the naive alternative of while True and provides flexibility of handling other low-level stuff such as Out-of-Memory reaper trying to kill the app (SIGKILL) or making a process go to sleep (SIGTSTP, SIGSTOP, SIGTTIN, SIGTTOU) or waking it up (SIGCONT).

wjandrea Over a year ago

"My recommendation is to actually let the user exit with Ctrl+C ... Programs that require a stronger signal to kill are annoying." -- I see your point, but it depends on the program. If OP's program calls other programs or keeps variables, exiting on Ctrl+C would be annoying, and it would be better to exit on Ctrl+D.

wim Over a year ago

Yes, well, that's why it should do its cleanup on SIGINT. If a program ignores Ctrl+C then it's a safe bet that a Ctrl+\ (SIGQUIT) may be coming soon after.. Correctly handling Ctrl+C doesn't preclude you from also supporting the Ctrl+D (EOF) usage for interactive programs.

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