Pylance seem to have difference with class decorators

[gintsmurans]

Hi!

This is a weird one. At first I thought there was an issue with extraPaths as those has been troubling, but turns out the issue was with my decorator.

When I use following decorator, pylance seem to be unable to find class properties:

def ignore_unknown_kwargs(cls: Type[Any]) -> Type[Any]:
    """
    Class decorator that modifies the __init__ method to ignore unknown keyword arguments.
    """
    originalInit = cls.__init__

    def newInit(self: Any, *args: list[Any], **kwargs: dict[str, Any]):
        # Filter out kwargs that are not properties of the class
        valid_kwargs = {k: v for k, v in kwargs.items() if hasattr(self, k)}
        originalInit(self, *args, **valid_kwargs)

    cls.__init__ = newInit  # type: ignore
    return cls

When I rewrite the decorator to this:

T = TypeVar("T", bound=Type[Any])

def ignore_unknown_kwargs() -> Callable[[T], T]:
    """
    Class decorator factory that modifies the __init__ method to ignore unknown keyword arguments.
    """

    def decorator(cls: T) -> T:
        originalInit = cls.__init__

        # @wraps(originalInit)
        def newInit(self: Any, *args: Any, **kwargs: Any):
            # Filter out kwargs that are not properties of the class
            valid_kwargs = {k: v for k, v in kwargs.items() if hasattr(self, k)}
            originalInit(self, *args, **valid_kwargs)

        cls.__init__ = newInit  # type: ignore
        return cls

    return decorator

Pylance seem to be able to find both correct and incorrect properties.

[rchiodo]

Thanks for the issue.

Do you have an example of where you're using this decorator?

[gintsmurans]

Thanks for the issue.

Do you have an example of where you're using this decorator?

Yes, lets say I have a class defined like this:

@ignore_unknown_kwargs
@dataclass
class RV4ProductionJobInfo(DBDataModel):
    """RV4ProductionJobInfo"""
    
    aField: int | None = field(...

And then in some other file I am doing something like this:

jobInfo = RV4ProductionJobInfo()
jobInfo.aField = 1 # This would be where properties are unknown. 

[erictraut]

Your decorator is erasing all type information because it returns a value of type type[Any].

If you want to preserve the signature of the decorated class (or its constructor), there are several ways to express this.

You could use a ParamSpec:

def ignore_unknown_kwargs[**P, R](cls: Callable[P, R]) -> Callable[P, R]: ...

Or you could use a regular type variable with a Callable upper bound:

def ignore_unknown_kwargs[T: Callable](cls: T) -> T: ...

Or you could use a regular type variable but specify that the pre-decorated type is a class:

def ignore_unknown_kwargs[T](cls: type[T]) -> type[T]: ...

There's (currently) no way in the Python type system to transform a signature in a way that adds an arbitrary number of untyped keyword arguments to a signature, so I don't see a way to reflect what your decorator is doing here.

[gintsmurans]

Well, that's interesting, because class it self defines those attributes that vscode cannot find. But I will not pretend to know inner workings of a type checker, so will close this for now and if somebody else have this same issue, they can find a solution in this thread.