Explaining Signal Module in Python

The aim of this page📝 solve the following encounter in Python code

signal.signal(signal.SIGINT, termination.handler)

• signal is a built-in module and something like an "OS event listener"

…a means through which a program can receive information from the Operating System. When the operating system receives certain events, it can pass that to programs in the form of signals.

— Python Signal Module — What are Signals and How to Create Them? — AskPython

SIGNAL TYPES / SIGNIT EXAMPLE

• you can listen for a defined set of signals; some are OS-specific
• for example, press CTRL + C → OS generates a signal → passes that to other programs
• for this combination, the event SIGINT is generated and passed
• on Win10, I am getting these 7 signals

>>> import signal3
>>> from pprint import pprint as pp

>>> pp(signal.valid_signals())
{<Signals.SIGINT: 2>,
 <Signals.SIGILL: 4>,
 <Signals.SIGFPE: 8>,
 <Signals.SIGSEGV: 11>,
 <Signals.SIGTERM: 15>,
 <Signals.SIGBREAK: 21>,
 <Signals.SIGABRT: 22>}

• ...INT stands for interrupt; this is what the docs say about this particular one

signal.SIGINT
Interrupt from keyboard (CTRL + C).

Default action is to raise KeyboardInterrupt.

signal.SIGTERM
Termination signal.

SIGNAL HANDLER

• The original example contains 2 arguments to the signal.signal() statement, the other is a handler, i.e. a function that should be called upon the firing of the signal.
• Note that

The handler is called with two arguments: the signal number (signum) and the current stack frame ( or a frame object)

• …however, I am not explicly using these 2 args
• yet, the handler function call will fail without them
• Finally, the handler will do what needs to be done when e.g. ctrl+c is pressed

EXAMPLE

import signal
def termination_handler(signo, frame):
    if input("~~> Interrupted ! Clean-up 'output.txt'? (y/N): ") == "y":
        open("output.txt", "w").close()
        exit("output deleted => bye")
    else:
        exit("output noT deleted => bye")

def main():
    signal.signal(signal.SIGINT, termination_handler)
    i = 0
    while True:
        i += 1
        print(i)
        with open("output.txt", "a") as f: f.write(str(i))

• In this particular example, the 🠉 has the same effect as catching the KeyboardInterrupt with a try/except block
• The 🠋 can contain a much smaller chunk of code, is, however, less economical/clean

def termination_handler():
    if input("~~> Interrupted ! Clean-up 'output.txt'? (y/N): ") == "y":
        open("output.txt", "w").close()
        exit("output deleted => bye")
    else:
        exit("output noT deleted => bye")


def main(i=0):
    try:
        while True:
            i += 1
            print(i)
            with open("output.txt", "a") as f: f.write(str(i))
    except KeyboardInterrupt:
        termination_handler()

LINKS

• signal — Set handlers for asynchronous events — Python 3.10.7 documentation
• Python Signal Module — What are Signals and How to Create Them? — AskPython