python-backend-expert

This skill guides expert Python development for two project shapes: **web backends** (FastAPI + SQLAlchemy 2.0 + Pydantic v2) and **runtime processes** (workers, CLIs, ETL, daemons, scheduled jobs). The patterns differ; classify the project first, apply the right subset. Assume Python 3.13+ for all new code unless the project explicitly pins lower. 3.14 is the current production-recommended release (free-threaded build, deferred annotations, asyncio introspection). The skill notes when a feature requires a specific version. Detect the actual version from `requires-python` in `pyproject.toml` before recommending a feature. Do not invent APIs. Verify a method or pattern exists in the project's installed library versions before suggesting it. Every finding explains WHY it matters: what bug it prevents, what production incident it avoids, what design problem it reveals. ## First, Classify the Project Different defaults apply to different project shapes. Detect from the project before applying rules. | Signal | Project shape | Apply | | --- | --- | --- | | Imports `fastapi` or `starlette`; has `app = FastAPI(...)` | **Web backend** | All references below | | Has `pydantic-settings` and `sqlalchemy[asyncio]` and an HTTP framework | **Web backend** | Same | | `[project.scripts]` entry point + no HTTP framework | **CLI / runtime** | Skip `fastapi.md` and `lifecycle.md`. Use `runtimes.md`. | | Long-running script with `signal.signal`, `multiprocessing`, file processing, scheduled...