Python / Uvicorn Fundamentals Interview Questions
What is the --factory flag in Uvicorn and when is it useful?
The --factory flag tells Uvicorn to treat the specified application path as a factory function - a callable that takes no arguments and returns an ASGI application - rather than as the application itself.
# Without --factory: APP is the application object directly uvicorn main:app # With --factory: create_app() is called to produce the ASGI app uvicorn main:create_app --factory # Example factory function: # main.py def create_app(): from fastapi import FastAPI app = FastAPI() @app.get("/") def root(): return {"status": "ok"} return app # The factory is called with no arguments and must return the ASGI app # Programmatic equivalent: uvicorn.run("main:create_app", factory=True, port=8000) # Why use a factory? # - Deferred initialisation (resources created only when workers start) # - Different config per environment (testing vs production) # - Integration with dependency injection frameworks # - Worker-level initialisation (each Gunicorn worker calls the factory separately)
Key use case - Gunicorn + factory: when using Gunicorn with multiple workers, each worker process calls the factory independently. This is important for resources that must be created per-process (like database connection pools) - a factory guarantees each worker initialises its own pool rather than sharing one created before forking.
What is an ASGI application factory in the context of the Uvicorn --factory flag?
Why is the factory pattern particularly useful with Gunicorn multi-worker deployments?
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