mypy error with union of callable and callable generator and typevar

Question

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def decorator(
        wrapped: Union[
            Callable[[], T],
            Callable[[], Generator[T, None, None]]
        ]
) -> Callable[[], T]:
    def wrapper():
        value = wrapped()
        if inspect.isgenerator(value):
            return next(value)
        else:
            return value
    return wrapper


@decorator
def foo() -> Generator[str, None, None]:
    yield "bar"

The above code produces the following error in mypy

error: Argument 1 to "decorator" has incompatible type "Callable[[], Generator[str, None, None]]"; expected "Callable[[], Generator[<nothing>, None, None]]"

Is this a limitation in mypy or am I doing something wrong?

Answer 1

It is a bug indeed. After debugging, I've discovered that if you explicitly set T generic:

U = Union[int, float, str] # your target types here
T = TypeVar('T', U, None)

or

T = TypeVar('T', None, Any)

It works as expected. If I set Any , it has to be the second parameter, but with union or scalar types it is being tested against the first argument.