Example of Callback Protocol

example.txt

$ python3.11 -m mypy main.py
main.py:19: error: Argument 2 to "add_something" has incompatible type "Callable[[str], int]"; expected "SomeCallback"  [arg-type]
Found 1 error in 1 file (checked 1 source file)

main.py

from some_lib import add_something


def add_one(x: int) -> int:
    return x + 1


def add_two(x: int) -> int:
    return x + 2


def bad_signature(x: str) -> int:
    return len(x)


def main() -> None:
    print(add_something(1, add_one))
    print(add_something(1, add_two))
    print(add_something(1, bad_signature))  # IDEs and mypy will catch this!



if __name__ == '__main__':
    main()

some_lib.py

from typing import Protocol


class SomeCallback(Protocol):
    def __call__(self, x: int) -> int:
        ...


def add_something(x: int, callback: SomeCallback) -> int:
    return callback(x)