How to compare types and fail if not the same

[kesitrifork]

I want to have pyright typecheck two types, I created this small examples, but I can't seem to make it work.

from typing import Callable, TypeVar, ParamSpec

P = ParamSpec("P")
R = TypeVar("R")

def ensure_return_type(fn: Callable[P, R], return_value: R) -> None:
    # If `return_value` doesn't match the annotated return type of `fn`,
    # Pyright will emit a type error before running the code.
    pass



def example_func() -> str:
    return "Hello"

ensure_return_type(example_func, "Some String")  # OK
ensure_return_type(example_func, 123)

[erictraut]

The type system doesn't provide a way to enforce "type equivalency". This may sound odd, but it makes more sense when you understand some of the theory behind type systems. This section of the typing spec may be useful to read.

When you use type variables in a function signature and call that function, a type checker will attempt to solve the type variables by finding some type that meets all of the constraints. In the example ensure_return_type(example_func, 123), R can be solved with the type str | int.

What you're trying to do in the above code is not a typical pattern, which explains why you're running into troubles. It's not clear to me from your code sample what you're trying to do.

Type equivalency is rarely what is desired in code. Instead, type compatibility is what is normally desired. For example, if you want to ensure that the value returned by the fn is addable to some other value, you could define your function as follows:

from _typeshed import SupportsAdd
from typing import Callable

def ensure_return_type_is_addable[**P, R1, R2](fn: Callable[P, R1], other_value: SupportsAdd[R1, R2]) -> None: ...

def example_func() -> str:
    return "Hello"

ensure_return_type_is_addable(example_func, "Some String")  # OK
ensure_return_type_is_addable(example_func, 123)  # Error