Configuration
How an application, its routers, middleware, lifespan, and settings are actually assembled.
Third-party documentation. This is independently authored analysis of the public FastAPI codebase — not the official docs, and not reviewed or endorsed by the FastAPI team.
Configuration
Application instance
A FastAPI app is created with FastAPI(...), accepting metadata (title, version, description) that flows directly into the generated OpenAPI schema, plus operational settings like docs_url, redoc_url, and openapi_url — any of which can be disabled entirely by passing None.
Routers
Larger applications are composed from multiple APIRouter instances, each independently testable, then mounted onto the main app with app.include_router(router, prefix="/items", tags=["items"]).
Middleware and CORS
Because FastAPI apps are Starlette apps, all of Starlette's ASGI middleware works unmodified — including CORSMiddleware, the standard way to configure cross-origin access.
Lifespan events
Startup/shutdown logic (opening a database pool, warming a cache) is expressed as an async context manager passed to FastAPI(lifespan=...), which has replaced the older @app.on_event("startup") decorators.
Settings management
The companion library pydantic-settings lets configuration be declared the same way as any other Pydantic model, reading from environment variables (and .env files) with the same validation guarantees as a request body.