Typing improvements in Middleware.

[pawelrubin]

Hi everyone!

Using Middleware or add_middleware hides the signature of the Middleware class being passed.

For example, the following code:

from starlette.applications import Starlette
from starlette.middleware import Middleware
from starlette.middleware.sessions import SessionMiddleware
from starlette.responses import JSONResponse
from starlette.routing import Route
from starlette.requests import Request


async def homepage(request: Request) -> JSONResponse:
    return JSONResponse({"hello": "world"})


app = Starlette(
    debug=True,
    routes=[
        Route("/", homepage),
    ],
    middleware=[
        Middleware(SessionMiddleware),
    ],
)

raises a Runtime exception:

TypeError: SessionMiddleware.__init__() missing 1 required positional argument: 'secret_key'

but no typing errors.

---

This can be improved by leveraging ParamSpec introduced in Python 3.10.
Here's a reference implementation in my fork: pawelrubin@0f4c8c5

I'd love to hear the Maintainers' and Community's feedback regarding this issue.

[adriangb]

Please submit a PR and we can look at it 😄

[pawelrubin]

Sure :) #2381