Pyright complains about typing of middleware argument in constructor

[martimors]

Error reported independently by both pyright and the pycharm type-checker, I assume it is also caught by mypy.

Repro example:

class AccessLogMiddleware(BaseHTTPMiddleware):
    async def dispatch(
        self, request: Request, call_next: Callable[[Request], Awaitable[Response]]
    ) -> Response:
        # Does nothing
        response = await call_next(request)
        return response

app = Starlette(middleware=[Middleware(AccessLogMiddleware)]

Errors reported by pyright:

error: Argument of type "type[AccessLogMiddleware]" cannot be assigned to parameter "cls" of type "type[_MiddlewareClass[P@__init__]]" in function "__init__"
error: Param spec "P@__init__" has no bound value

[Kludex]

I can't reproduce it. Are you in the latest pyright version?

[koldakov]

Looks like I'm facing the same issue, even without any extra instruments.

The problem is that in the commit 866a15f there was a change for type hinting from

middleware_class: type,

to

middleware_class: typing.Type[_MiddlewareClass[P]],

So it waits for middlewares to be inherited from _MiddlewareClass, what is wrong I believe.

From the docs:

from starlette.applications import Starlette
from starlette.middleware.base import BaseHTTPMiddleware


class CustomMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):
        response = await call_next(request)
        response.headers['Custom-Header'] = 'Example'
        return response


app = Starlette()
app.add_middleware(CustomMiddleware)

Gives:

Expected type 'Type[_MiddlewareClass]' (matched generic type 'Type[_MiddlewareClass[ParamSpec("P")]]'), got 'Type[CustomMiddleware]' instead

Environment:

python==3.12.0
starlette==0.36.3

[btakita]

Looks like I'm facing the same issue, even without any extra instruments.

I also see this issue using a Jetbrains editor, such as Idea or Pycharm.

Expected type 'Type[_MiddlewareClass[ParamSpec("P")]]', got 'Type[CORSMiddleware]' instead 

Note that the middleware is starlette.cors.CORSMiddleware.

[just-maiyak]

I have a similar error popping up, I've made an Issue over on asgi-loggers' repo documenting my case. Since I posted, I figured out it started since commit 866a15.

[AdmWinther]

I am facing the same issue in FastAPI. I have made this middleware

from starlette.middleware.base import BaseHTTPMiddleware
class MyCORSMiddleware(BaseHTTPMiddleware):
    async def dispatch(self, request, call_next):

and when I use it like this:

app.add_middleware(MyCORSMiddleware)

I get the warning related to type mismatch.

[M-Studley]

Any solutions? I hate doing #noqa in my code

[koldakov]

Any solutions? I hate doing #noqa in my code

For now now, I described why this happens here

Or you can use cast, but it's better to use noqa )