Python3 Web

---
name: python3-web
description: Python web and API development enforcing strict route/domain/data layer separation, Pydantic v2 strict request-response models, edge-resolved auth, and async-safe HTTP clients. Use when working with FastAPI, Starlette, Django, Flask, HTTP endpoints, request models, authentication flows, async handlers, or any Python web framework task.
user-invocable: false
---

# 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

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