Agent Skills: Python Web

Python Web

Load python3-core for standing defaults. Load python3-typing for request/response modeling. Load python3-testing for endpoint and auth tests.

Quality Checklist

• [ ] Route handlers contain NO business logic — only validation, delegation, response mapping
• [ ] No framework objects (Request, Response, ORM models) leak into domain layer
• [ ] model_config = {"strict": True} on all request models
• [ ] Separate request and response models per endpoint — never reuse ORM models as API schemas
• [ ] Auth resolved at edge; domain functions receive typed identity, never raw tokens
• [ ] Domain exceptions mapped to HTTP status codes at boundary, not scattered through handlers
• [ ] Async handlers use httpx.AsyncClient — never requests (blocks event loop)
• [ ] Background tasks hold no references to request-scoped objects

Gotchas

| Trap | What to do instead | |---|---| | FastAPI dep ordering assumptions | Dependencies resolve by graph, not definition order — chain explicitly if order matters | | Pydantic v1/v2 method mixing | v2: model_dump(), model_dump_json(), model_validate() — never .dict() / .json() | | import requests in async handler | requests blocks event loop; use httpx.AsyncClient with async with | | Django ORM in async views | Bare Model.objects.get() blocks; use aget() (Django 4.1+) or sync_to_async | | allow_origins=["*"] | Enumerate specific origins — * disables credential support, security risk | | Single model for request + response + DB | Three separate models: CreateUserRequest, UserResponse, UserDB |

Decision Table: Sync vs Async

| Scenario | Use | Why | |---|---|---| | CPU-bound processing | Sync + thread pool | async doesn't help CPU work | | Multiple external API calls | Async + httpx.AsyncClient | Concurrent I/O without threads | | Simple CRUD with sync ORM | Sync handlers | Async adds complexity with no benefit | | WebSocket connections | Async (required) | Inherently long-lived |

Layer Separation

HTTP Layer (routes) → validated typed request models
Domain Layer (logic) → typed domain objects
Data Layer (repo/ORM)