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pythonpython-decoratorsmypy
HIT_girl
HIT_girl

Reputation: 875

MyPy fails to recognize decorated function's return type

I have an odd mypy failure when using decorators from an installed (via pip install) package. The same decorator written within my app code, works just fine.

I have a function like such:

def _subscription_entity(self) -> Optional[SubscriptionEntity]:
   return get_subscription(...)  # get_subscription() is a decorated function

MyPy is failing with an error: Returning Any from function declared to return Optional[SubscriptionEntity]

However, if I copy the whole decorator code, and place it within my app code (in a separate file, and importing that file instead of package installed one), all works as expected. No errors. I have also tested with changing _subscription_entity signature to return an int and I get an expected error Returning Optional[SubscriptionEntity] from function declared to return int

Why would mypy fail when decorator code lives inside a package, but not when lives inside app code ?

The simplified decorator is as follows

F = TypeVar('F', bound=Callable[..., Any])

def test_cache(wrapped_func: F) -> F:

    @wraps(wrapped_func)
    def decorated_func(*args: Any, **kwargs: Any)-> F:
        return _handle(wrapped_func, *args, **kwargs)

    return cast(F, decorated_func)

def _handle( wrapped_func: Callable, *args: Any, **kwargs: Any) -> F:
    return wrapped_func(*args, **kwargs)

The decorated function is:

@test_cache
def get_subscription(cls, request: GetSubscriptionRequest) -> Optional[SubscriptionEntity]:
    ....

Upvotes: 2

Views: 2328

Answers (1)

HIT_girl
HIT_girl

Reputation: 875

Turns out my imported package didn't have the right metadata to let MyPy know that package has typing. https://mypy.readthedocs.io/en/stable/installed_packages.html?highlight=py.typed#making-pep-561-compatible-packages After adding py.typed to package_data in setup all worked OK with imported package

Upvotes: 1

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