Add ContentType generic to Response

[Kludex]

• Retry of Make responses generic #1534.

At the time that PR was implemented the default= parameter was not available on TypeVar, and mypy didn't support it.

Now it's possible to achieve what that PR intended.

[Kludex]

default= was introduced in this version: https://typing-extensions.readthedocs.io/en/latest/#typing_extensions.TypeVar

[Kludex]

Last release supports default=.

[adriangb]

I think JsonResponse and others don’t actually accept Any, they accept a limited set of types. Is that codified in this PR? If it is I’m missing it.

[adriangb]

Just to confirm, this is a breaking change right?

[Kludex]

Oh yeah, it is...

[Kludex]

I think JsonResponse and others don’t actually accept Any, they accept a limited set of types. Is that codified in this PR? If it is I’m missing it.

You are right, but I'm not changing that in this PR. And I think there was an attempt to change the type, but it failed, and we had to revert.

[adriangb]

It seems to me like the reality is that the default should be str | bytes given what Response accepts. Then subclasses like StreamingResponse should override this generic with whatever they accept. If we make the default typing.Any then Response is not correctly typed. Since we're setting the default in this PR we might as well choose an appropriate value.

[Kludex]

I'm not changing this in this PR. There was a previous attempt to change the type to str | bytes | None, and it was reverted - I don't remember the reason.

[adriangb]

Ok. It would be good to find that reason.

[Kludex]

#2242

[adriangb]

Ok I see. So yes the right thing to do would be to make a BaseResponse and make Response just one of the many concrete classes:

_ResponseDataT = TypeVar('_ResponseDataT')

class BaseResponse(Generic[_ResponseDataT]):
    def render(self, data: _ResponseDataT | None) -> bytes:
        raise NotImplementedError

class Response(BaseResponse[str | bytes]):
    ...

_JsonData = str | bytes | float | int | None | ...

class JsonResponse(BaseResponse[_JsonData]):
    ...

[Kludex]

But the Response class does have the .render() method... So I don't see how I'd that? I can't make the classes stop inheriting Response, because that would be a breaking change.

[abersheeran]

Yes, this is also what I think is unreasonable about Starlette. For example, StreamResponse and FileReponse do not use render, but they inherit this method.

[Kludex]

Are you suggesting adding a breaking change then?

[abersheeran]

Yes. 😊

[adriangb]

Can we do some naming to make it a non-breaking change or minimize the breakage? We could leave the render() method on the base class but mark it as deprecated vi a warning. Adding a BaseResponse class is not a breaking change. I think removing the render() method from eg StreamingResponse (because it inherits from BaseResponse and not Response) would also be okay.