I have the following python code:

from typing import TypeVar, Callable, Optional
import os

T = TypeVar("T")

def config(key: str, default: Optional[T] = None, cast: Callable[[str], T] = lambda x: x) -> T:
    value = os.environ.get(key)
    if value is None:
        if default is None:
            raise KeyError()
        return default
    return cast(value)

When I run it through pyright it doesn’t give me any errors. But mypy give me two errors:

error: Incompatible default for argument "cast" (default has type "Callable[[str], str]", argument has type "Callable[[str], T]")  [assignment]
error: Incompatible return value type (got "str", expected "T")  [return-value]

I also tried making cast optional with a default value of None then in the function body I check if cast is None, I return value as is otherwise I run it through cast. This resolves the first error but the second error persists.

Is there a way to correctly type this function in mypy without loosening the types with something like Any or returning T | str?