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You also need to add self: AppType

Good point, thanks @adriangb! Trying it locally I realized I had to redefine AppType to be bound to FastAPI as well.

Hmm I’m surprised the bound=Starlette should work and the fact that you have self: AppType should make the AppType always match self which is going to be a FastAPI instance.

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I'm trying with this example:

from collections.abc import AsyncGenerator
from contextlib import asynccontextmanager

from fastapi import FastAPI


@asynccontextmanager
async def lifespan(_: FastAPI) -> AsyncGenerator[None, None]:
    ...
    yield
    ...


app = FastAPI(lifespan=lifespan)

And I get this error:

mypy sand.py
fastapi/applications.py:112: error: "AppType" has no attribute "openapi_schema"
fastapi/applications.py:113: error: "AppType" has no attribute "openapi_url"
fastapi/applications.py:114: error: "AppType" has no attribute "title"
fastapi/applications.py:115: error: "AppType" has no attribute "version"
fastapi/applications.py:126: error: "AppType" has no attribute "dependency_overrides"
fastapi/applications.py:153: error: "AppType" has no attribute "setup"
Found 6 errors in 1 file (checked 1 source file)

...I guess it would be fine for external things to have AppType be bound to Starlette, but for the internals of FastAPI it seems to be a problem. But I guess it would be fine then to have the custom AppType here.

Weird, we even have a test in Starlette: https://github.com/encode/starlette/blob/8965f8849977b5752e415722a83aa00049f80ff3/tests/test_applications.py#L539-L550

Ah, yeah, the error is not in the lifespan, but that the class doesn't have the custom attributes FastAPI defines.