conventions

• SKILL.mdcore · 3,522 bytes
• SKILL.mdcore · 3,522 bytes

# Python

House conventions for Python 3.11+. Apply them to code you are writing or changing — don't refactor untouched files to match unless asked.

## Conventions

- **Type every public signature** and keep `mypy --strict` green. Types on internals are optional; types at the boundary are what stop a caller passing the wrong thing.
- **Built-in generics and `X | None`** — `list[str]`, `dict[str, int]`, `str | None`. `typing.List` and `Optional[str]` are the pre-3.10 spelling and only cost an import.
- **`Protocol` over ABC inheritance.** Structural typing lets any correctly-shaped object satisfy the contract — including a test double — with no inheritance tree to maintain.
- **Dataclasses for data**, `slots=True` and `frozen=True` where they fit. They generate `__init__`, `__repr__`, and `__eq__` correctly; a hand-written `__init__` is where field drift starts. Pydantic is for validation and (de)serialisation at a boundary, not for plain records.
- **`pathlib`, not `os.path`.** Operator joins can't silently produce a wrong path from a stray separator.
- **Never a mutable default argument.** `def f(items=[])` shares one list across every call — a bug that only appears on the second call. Default to `None` and build inside.
- **`asyncio.TaskGroup` over bare `gather`** (3.11+): it cancels siblings on failure and reports via `ExceptionGroup`, so a crashed task can't leave the rest running detached. Use `async with asyncio.timeout(n)` for deadlines.
- **Hold a reference to every `create_task`.** The event loop only keeps a weak reference, so a fire-and-forget task can be garbage-collected mid-execution and simply vanish — no error, no result. Keep them in a `set` and discard on completion: