Python: type annotate class decorator return type

Question

How to annotate a class decorator that extends a class with additional members?

I have the following example where my class decorator inserts an async new function that calls the provided class' ainit . This code runs correctly, but the type returned by optimizer is the original type, and new is missing. How can I improve annotation, if possible?

AinitArguments = ParamSpec("AinitArguments")

class Optimizerable(Protocol):
    ainit: Callable[AinitArguments, None]


T = TypeVar("T", bound=Optimizerable)


def optimizer(cls: Type[T]) -> Type[T]:
    @classmethod
    async def new(cls: Type[T], *args, **kwargs) -> T:
        self = cls.__new__(cls)
        await self.ainit(*args, **kwargs)
        return self

    setattr(cls, "new", new)
    return cls

@optimizer
class Myclass:
    async def ainit(self, a: int, b: float) -> None:
        pass # stub

Answer 1

Long story short, you can't really do this yet https://github.com/python/typing/issues/213

Python doesn't have a native type for an Intersection to represent a concrete class implementing two bases (either explicitly through inheritance or duck-typing like with protocol).

What you want would look like this (which just isnt possible yet):

class Optimizerable(Protocol):
    ainit: Callable[AinitArguments, None]

T = TypeVar("T", bound=Optimizerable)

class Optimized(Protocol[T]):
    async def new(cls: Type[T], *args, **kwargs) -> T:
        ...

def optimizer(cls: Type[T]) -> Type[Intersection[T, Optimized[T]]]:
    ...

I would either refactor this code to use a Mixin to add the method, or simply keep new as a stateless function, which avoids complex class structures entirely:

T = TypeVar("T", bound=Optimizerable)


async def optimized_new(cls: Type[T], *args, **kwargs) -> T:
    self = cls.__new__(cls)
    await self.ainit(*args, **kwargs)
    return self