[BUG][DOCUMENTATION] Sub application startup event never triggered

Describe the bug

Startup event handlers for sub-applications never trigger.

To Reproduce

from fastapi import FastAPI


sub_app = FastAPI(openapi_prefix="/subapp")


@sub_app.on_event("startup")
async def sub_app_startup():
    print("++++ Sub-app startup event")  # Never fires


app = FastAPI()


@app.on_event("startup")
async def app_startup():
    print("++++ App startup event")


app.mount("/subapp", sub_app)


if __name__ == "__main__":
    import uvicorn

    uvicorn.run(app, port=8000, host="0.0.0.0")  # nosec

Running the above shows:

$ python subapp_events.py
INFO:     Started server process [38462]
INFO:     Uvicorn running on http://0.0.0.0:8000 (Press CTRL+C to quit)
INFO:     Waiting for application startup.
++++ App startup event
INFO:     Application startup complete.

Notice that sub_app_startup() is never run.

Expected behavior

I would expect the sub-application startup event handler to also run (although I am not sure what ordering I would expect).

Environment

• OS: macOS
• FastAPI Version: 0.45.0
• Python version: 3.7.5

Additional context

N/A

[tiangolo]

@ClericPy it would have to be implemented in Starlette: https://github.com/encode/starlette/issues/649

[jonathanunderwood]

Digging in to this a bit more, I can see that FastAPI merely exposes the Starlette lifecycle events and doesn’t do anything special about them.

In addition, this issue has been raised previously against Starlette.

Since it’s deemed out-of-scope by Starlette developers, I think the only action here would be to document this in the FastAPI docs i.e. call out explicitly that sub-application event handlers are ignored.