FastAPI: typed, async, auto-documented

FastAPI is Flask's modern successor — same simplicity, but type hints do triple duty:

1. Editor autocomplete — IDEs know the shape of your data.
2. Runtime validation — Pydantic rejects bad payloads with a clear 422 error.
3. Auto-generated OpenAPI docs — visit /docs for a live Swagger UI, /redoc for ReDoc.

Core concepts

Pydantic models = your API contract

class TodoIn(BaseModel):
    title: str = Field(min_length=1, max_length=200)
    done: bool = False

If a client POSTs {"title": ""}, FastAPI auto-rejects with 422. You don't write validation code.

Response models

response_model=Todo filters output — you can't leak fields you didn't declare.

HTTPException

Raise HTTPException(404, "Not found") for typed error responses.

Async when you need it

Any handler can be async def — great for calling other APIs or an async DB driver like asyncpg.

Run it

uvicorn main:app --reload

Then open http://localhost:8000/docs.

Try it (mentally, before running locally)

1. Add a DELETE /todos/{todo_id} endpoint.
2. Add a query param ?done=true to GET /todos that filters.