What is signal type?

[dineshbvadhia]

The code is to exit a program when ^C is pressed. How to type this code?

import signal

signal.signal(signal.SIGINT, signal_handler)

def signal_handler(signal, frame): -> None
    print('program terminated')
    return

[Akuli]

In typeshed, stdlib/signal.pyi defines signal.signal() here: https://github.com/python/typeshed/blob/3a22bf2411c7357bffb2fe1dc675486a8da97177/stdlib/signal.pyi#L68

Your function is passed as a _HANDLER, and the definition of _HANDLER just above is: Union[Callable[[int, Optional[FrameType]], Any], int, Handlers, None]

You are passing in the function, so we can ignore everything except the Callable, and your function should match Callable[[int, Optional[FrameType]], Any]. This means a function that takes an int as first argument, and Optional[FrameType] as second argument, and can return anything it wants.

import typing
from types import FrameType

def signal_handler(signal: int, frame: Optional[FrameType]) -> None:
    print('program terminated')

[dineshbvadhia]

Thank-you. My first time here: So, is process to check typeshed first? Secondly, are questions like this for General Discussion or Q&A?

[Akuli]

Checking typeshed works. If you use a smart enough editor, you will likely see some kind of popup when you e.g. bring your mouse over signal.signal, and those typically display information from typeshed. Sometimes that's enough and you don't have to dig into typeshed yourself.

I don't know whether General Discussion or Q&A is more appropriate, but I'd say it doesn't matter much. I think Q&A makes more sense, as it's clearly a question that you expect someone to be able to just answer.

[hauntsaninja]

Often times you can just add your best guess at a type, and if it's wrong your type checker hopefully has a friendly error

[dineshbvadhia]

Started type checking with vscode but it is good to understand why for the tricky ones. Back to my editor to finish manually :)

[dineshbvadhia]

Hi. Back again. The code

import signal
import typing
from types import FrameType

signal.signal(signal.SIGINT, signal_handler)    # line 95

def signal_handler(signal: int, frame: Optional[FrameType]) -> None:
    print('program terminated')

causes the error:

Traceback (most recent call last):
  File "C:\....py", line 95, in <module>
    signal.signal(signal.SIGINT, signal_handler)
NameError: name 'signal_handler' is not defined

What is the problem?

[Gobot1234]

You are attempting to use access the signal_handler name before it's actually defined. You need to move the signal.signal call to after you define the function.