Python Typing Optional String Callback

README.md

The Python here works just fine, but mypy doesn't like it...

example.py:14: error: Argument 1 to "callback" has incompatible type "Optional[str]"; expected "str"

The if url is None: conditional should prevent the url in the else block from ever being None, but maybe it doesn't work like that? Or I'm just wrong...

example.py

from typing import Callable, Optional


def callback(url: str, value: int) -> None:
    print(f"Did something with {url} and {value}")


def build_callback(url: Optional[str]) -> Callable[[int], None]:
    if url is None:
        def cb(value: int) -> None:
            pass
    else:
        def cb(value: int) -> None:
            callback(url, value)

    return cb


build_callback(None)(42)  # noop
build_callback('https://lol.wut')(42)

The answer here is kind of ugly... The mypy type checker doesn't appear to be aware of the context for url across the function boundary of cb, so it thinks url still has type Optional[str] even though that's made impossible by the conditional. This can be "resolved" by defining the callback for the str case as

def cb(value: int) -> None:
    # This assert forces the type checker to be aware of the true type of `url`.
    assert url is not None
    callback(url, value)

Gross, but it works and the type checker is satisfied. Thanks again to @RadicalZephyr for the assist.

I'm not so keen on the assert in production code here.

I prefer to have an if starement
if url is None:
then raise some blank error
raise ValueError

as you said, its impossible for this error to actually get raised, but it should satisfy mypy, and I think it looks a bit nicer than the assertion :)