How do you create and run a minimal FastAPI application?

A FastAPI app needs just a few lines. You create a FastAPI() instance and decorate Python functions with HTTP method decorators. The app is served by an ASGI server — Uvicorn is the standard choice.

# main.py
from fastapi import FastAPI

app = FastAPI()

@app.get("/")
def read_root():
    return {"message": "Hello, FastAPI!"}

@app.get("/items/{item_id}")
def read_item(item_id: int, q: str | None = None):
    return {"item_id": item_id, "q": q}

# Run the server
uvicorn main:app --reload

# main  = the module (main.py)
# app   = the FastAPI() instance variable
# --reload = auto-restart on code changes (dev only)

After starting, visit http://127.0.0.1:8000/docs for the interactive Swagger UI, which FastAPI generates automatically from your code. The --reload flag is for development only — never use it in production.

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