Fix issue with middleware args passing

[uriyyo]

Summary

A feature introduced in #2381 doesn't allow to pass args inside middleware, here is an example:

from starlette.applications import Starlette
from starlette.types import ASGIApp, Receive, Scope, Send
from starlette.testclient import TestClient


class Middleware:
    def __init__(self, app: ASGIApp, arg: str) -> None:
        self.app = app
        self.arg = arg

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        self.app(scope, receive, send)


app = Starlette()
app.add_middleware(Middleware, "foo")

client = TestClient(app)
response = client.get("/")

TypeError: Middleware.__init__() got multiple values for argument 'app'

Also, make type annotation for middleware less strict, remove restriction for middleware to be class, and make it callable that returns ASGI application, it's a fully backward-compatible change.

from starlette.applications import Starlette
from starlette.responses import JSONResponse
from starlette.types import ASGIApp, Receive, Scope, Send
from starlette.testclient import TestClient


def forbid_http_middleware(app: ASGIApp) -> ASGIApp:
    async def middleware(scope: Scope, receive: Receive, send: Send) -> None:
        if scope["type"] == "http":
            _response = JSONResponse({"detail": "Forbidden"}, status_code=403)
            await _response(scope, receive, send)
        else:
            await app(scope, receive, send)

    return middleware


app = Starlette()
app.add_middleware(forbid_http_middleware)

client = TestClient(app)
response = client.get("/")

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.

[adriangb]

Is this safe to do? I guess the point is that the self.cls isn't necessarily a class, it may be a function, but even functions have __name__ defined. Maybe we should do getattr(self.cls, "__name__", "") or something to make sure we don't error in any case?

[adriangb]

(alternatively we could add __name__ to the protocol above 😆 )

[uriyyo]

@adriangb Thanks for suggestion, changed it to be getattr with default value call.

[adriangb]

Thanks! Are there any actual changes to the reprs? I'm not seeing why that code had to change (aside from what I pointed out in this comment)

[uriyyo]

There were no actual changes to the repr. I changed it only because of mypy error.

[adriangb]

Thanks!

[arni-xalgo-io]

Upgrading from 0.41.2 to 0.41.3 gives me the following type error

REDACTED/api.py:104: error: Missing positional arguments "scope", "receive", "send" in call to "add_middleware" of "Starlette"  [call-arg]
REDACTED/api.py:104: error: Argument 1 to "add_middleware" of "Starlette" has incompatible type "Type[ExceptionHandlerMiddleware]"; expected "_MiddlewareFactory[[MutableMapping[str, Any], Callable[[], Awaitable[MutableMapping[str, Any]]], Callable[[MutableMapping[str, Any]], Awaitable[None]]]]"  [arg-type]
REDACTED/api.py:104: note: Following member(s) of "ExceptionHandlerMiddleware" have conflicts:
REDACTED/api.py:104: note:     Expected:
REDACTED/api.py:104: note:         def __call__(app: Callable[[MutableMapping[str, Any], Callable[[], Awaitable[MutableMapping[str, Any]]], Callable[[MutableMapping[str, Any]], Awaitable[None]]], Awaitable[None]], scope: MutableMapping[str, Any], receive: Callable[[], Awaitable[MutableMapping[str, Any]]], send: Callable[[MutableMapping[str, Any]], Awaitable[None]]) -> Callable[[MutableMapping[str, Any], Callable[[], Awaitable[MutableMapping[str, Any]]], Callable[[MutableMapping[str, Any]], Awaitable[None]]], Awaitable[None]]
REDACTED/api.py:104: note:     Got:
REDACTED/api.py:104: note:         def __call__(self: ExceptionHandlerMiddleware, scope: MutableMapping[str, Any], receive: Callable[[], Awaitable[MutableMapping[str, Any]]], send: Callable[[MutableMapping[str, Any]], Awaitable[None]]) -> Coroutine[Any, Any, None]
Found 2 errors in 1 file (checked 44 source files)

✕ mypy failed.

for the following line of code

app.add_middleware(ExceptionHandlerMiddleware)

where app is a FastAPI app.

The middleware is like this

from starlette.types import ASGIApp, Receive, Scope, Send

class ExceptionHandlerMiddleware:
    def __init__(self, app: ASGIApp):
        self.app = app

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        # Implementation here
        ...

I'm guessing it's a regression resulting from this PR.

[uriyyo]

@arni-xalgo-io There is discussion about this issue - #2757