How to typehint an attribute of a generic type?

[FeryET]

Hi.

I'm trying to write a decorator for my functions that gets the function result type as its input, and then returns only the required attribute.

Something similar to this idea, look at the Attribute typing class that I have used.

Params = ParamSpec("Params")
T = TypeVar("T")

def dedecorator(key: str):
      def decorator(func: Callable[Params, T]) -> Callable[Params, Attribute[T, key]]:
             def wrapper(*args: Params.args, **kwargs: Params.kwargs) -> Attribute[T, key]:
                    return getattr(func(*args, **kwargs), key)
             return wrapper
      return decorator

Is something possible in Python as of now?

[erictraut]

No, that's not possible in the type system today. If there's a specific attribute that you're interested in, you could define a generic protocol class, like this:

Code sample in pyright playground

from typing import Callable, Protocol

class SupportsFoo[T](Protocol):
    """ Structural type for a class that supports a read/write "foo" parameter """
    foo: T

def decorator[**Params, T](func: Callable[Params, SupportsFoo[T]]) -> Callable[Params, T]:
    def wrapper(*args: Params.args, **kwargs: Params.kwargs) -> T:
        return getattr(func(*args, **kwargs), 'foo')
    return wrapper


class FooImpl:
    foo: int

@decorator
def func() -> FooImpl:
    ...

[Daverball]

There is also a mypy plugin that may help you get there https://pypi.org/project/mypy-extras/ which has an AttrOf special type, although it expects a Literal for the second argument, so it probably wouldn't work, unless you also wrote a mypy plugin specifically for this decorator, in order to resolve the first argument to a specific Literal instead of str (maybe using LiteralString instead of str would accomplish this, although I'm not sure it actually would).