Consider the following minimal example in which we have a function foo that can be either called with one int and one string or with two strings: from typing import overload @overload def foo(x: int, y: str) -> str: ... @overload def foo(x: str, y: int) -> str: ... @overload def foo(x: str, y: str) -> int: ... def foo(x, y): if isinstance(x, int): return str(x - 2) + y if isinstance(y, int): return str(y - 2) + x return int(x + y) While the implementation of foo should be fine, it is not checked by mypy. So if we should have any type error in the function body, it will not be caught. So here is my question: Why doesn't mypy simply check the consistency of every signature with the implementation? And how could we make mypy check every signature? Clearly, we can't annotate the implementation like this: def foo(x: str | int, y: str | int) -> str | int: # mypy complains rightfully pass Neither do I see how we could a TypeVar to resolve the issue. So what should we do? Some discussion of this can be found in this GitHub issue and in this question. Neither provides a solution for the problem I have described or a satisfying justification for why type checking is "too challenging" for overloaded functions. Continue reading...
