Merging/Modifying Types

I have a decorator that modifies a class at runtime, kind of like a dataclass:

@modify
class Something:
    ...

Let's say modify adds foo onto Something, so after calling modify you could use it like so:

@modify
class Something
    ...

bar = Something.foo()

dataclass_transform doesn't support this kind of behavior at all. So, how would an API like this work?

I don't see why it would be difficult to implement some sort of Merge type, that takes in a type and "applies"" a Protocol to it. For the above example, it would work like this:

T = TypeVar("T")

class FooProtocol(Protocol):
    @classmethod
    def foo() -> None:
        ...

def modify(tp: type[T]) -> Merge[T, FooProtocol]:
    tp.foo = ...  # type: ignore
    return tp

@modify
class Something:
    ...

bar = Something.foo()

In the above example, ideally it would just sort of mark that the T is now compatible with FooProtocol, but a type: ignore is used inside of modify because it would probably be a mess to try and come up with some sort of API for actually converting a T to Merge[T, ...].

TL;DR A type that marks another type as being compatible with a protocol could be handy for making dataclass-like decorators.

Start by reviewing the proposed Merge type and the dataclass_transform limitation described in the issue. Determine whether the typing specification has an appropriate place for this behavior and what compatibility and decorator semantics would need to be defined; the issue provides no files, tests, or concrete implementation entry point.