Correct pattern for only-once setup with eager app loading

[tlinhart]

Hi, after merging this PR (#2919) when reloader is active, the app is loaded both in the parent process and in the worker, if I understand it correctly. In my setup I use an app factory pattern and in the main file create the app. Also, if DEBUG=1 env variable is set, I attach debugpy. However, now that the app is loaded twice, it leads to "address already in use" problem. What's the best way to solve this particular case?

[goutamadwant]

PR #2919 deliberately imports the module once in the reloader parent for fail-fast validation, then imports it again in the spawned server process. An app factory prevents the parent from calling the factory, but it cannot make module-level side effects run only in the worker.

Keep the imported module side-effect-free and move debugpy.listen() into ASGI lifespan, which only the server process runs:

from contextlib import asynccontextmanager
import os
import debugpy
from fastapi import FastAPI

@asynccontextmanager
async def lifespan(app):
    if os.getenv("DEBUG") == "1":
        debugpy.listen(("127.0.0.1", 5678))
    yield

def create_app():
    return FastAPI(lifespan=lifespan)

if __name__ == "__main__":
    import uvicorn
    uvicorn.run("main:create_app", factory=True, reload=True)

Uvicorn's reload path terminates and joins the old worker before starting the replacement, so the port is released between reloads. A fixed debugger port still cannot be shared by multiple simultaneous --workers; use one worker in this debug setup or assign per-worker ports.