[Python] How to add a middleware before the application starts up in FastAPI?

Using Lifespan Events and Middleware, I would like to add a middleware inside an asynccontextmanager (at application startup). It should look something like this:

@asynccontextmanager
async def lifespan(app: FastAPI):
if some_condition:
@app.middleware("http")
async def my_middleware(request: Request, call_next):
response = await call_next(request)
return response
yield
pass


app = FastAPI(lifespan=lifespan)


However, the example above raises the following error:

RuntimeError: Cannot add middleware after an application has started


But, in the beginning of Lifespan Events documentation, it is mentioned that:

You can define logic (code) that should be executed before the application starts up. This means that this code will be executed once, before the application starts receiving requests.
​

How should I approach this?

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