How do I ensure that function arguments must be of the same type?

[ashenstrata]

I want to ensure on a type checker level that 2 arguments of a function must always be the same exact type.

V = TypeVar("V")


def some_function(foo: V, bar: V) -> V:
    ...

I expect an error to be shown when calling the function with arguments of different types:

res = some_function(1, "s")

However, it doesn't work. The V type just gets broader (int | str) and there is no error.

As I understand, this is intended behavior. But how can I achieve what I need?

[trendels]

But how can I achieve what I need?

I don't think this is possible.

If you turn the function into a method of a generic class, you can enforce that both arguments are of the same type, but you also have to specify the type:

V = TypeVar("V")

class MyClass(Generic[V]):
    @staticmethod
    def some_method(foo: V, bar: V) -> V: ...

res = MyClass[int].some_method(1, "s")  # this is now a type error

With the proposed PEP for subscriptable functions, this would probably also work.

res = some_function[int](1, "s")  # should be a type error