Middleware _MiddlewareClass mypy typing PR2381

[ShedPlant]

I have some custom middleware a bit like

class TraceTokenLoggerMiddleware:
    def __init__(self, app, logger: IAsyncLogger):
        self._app = app
        self._async_logger = logger

    async def __call__(self, scope: dict, receive, send):
         # do something
        pass

Then creating my app:

App(
    ...
    middleware=[
            Middleware(TraceTokenLoggerMiddleware, logger=async_logger)
    ]
)

My project uses mypy to test typing, and this passed using 0.32.0

#2381 changed starlette.middleware.Middleware's first argument from type Any to type[_MiddlewareClass[P]]

I upgraded to mypy 1.10.0 and starlette 0.37.2.

Now when I run mypy I get an error like

error: Argument 1 to "Middleware" has incompatible type "type[TraceTokenLoggerMiddleware]"; expected "type[_MiddlewareClass[[AsyncLogger]]]"  [arg-type]

Please advise how to stop this error or if a fix is needed in starlette.

It seems like I ought to make MyMiddleware inherit from _MiddlewareClass?! But it's private so I shouldn't do that!

[ShedPlant]

FWIW if I do define class TraceTokenLoggerMiddleware(_MiddlewareClass): and change the type hints of __call__, then mypy stops complaining.

So I suggest _MiddlewareClass be made public, renamed to MiddlewareAbstractClass or something.

[pawelrubin]

The _MiddlewareClass is a Protocol meant for structural subtyping, not for subclassing. Check out this SO answer for a neat explanation.

I see the point in making the type "public" though. Perhaps we could add it to starlette.types? WDYT @adriangb?

[adriangb]

I don’t think that’s really necessary. You can always import it as private, define the class, fix any type errors then remove the import / subclass.

[ShedPlant]

@pawelrubin and @adriangb , are you able to comment / help please?

[pawelrubin]

You should be able to fix the issue by providing complete type annotations to your TraceTokenLoggerMiddleware:

from typing import Any

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


IAsyncLogger = Any  # whatever the type is


class TraceTokenLoggerMiddleware:
    def __init__(self, app: ASGIApp, logger: IAsyncLogger) -> None:
        self._app = app
        self._async_logger = logger

    async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
        # do something
        pass

async_logger: IAsyncLogger = NotImplemented

Middleware(TraceTokenLoggerMiddleware, logger=async_logger)  # no errors here

[ShedPlant]

Thanks, I can confirm mypy doesn't complain anymore if I do that.

Although, my IDE (PyCharm Community Edition) does

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