conventions

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.

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:
```
_tasks: set[asyncio.Task[None]] = set()
```

def spawn(coro: Coroutine[None, None, None]) -> None: task = asyncio.create_task(coro) _tasks.add(task) task.add_done_callback(_tasks.discard)


- **Never a bare `except:`** — it swallows `KeyboardInterrupt` and `SystemExit`. Catch what you can actually handle.
- **Google-style docstrings** on public functions and classes, carrying intent rather than a restatement of the signature. The `documentation` skill has the full rule.
- **pytest: fixtures for setup, `parametrize` for cases.** A loop inside one test reports a single failure and hides which case broke; `parametrize` names each one.
- **Poetry and `pyproject.toml`** for packaging, ruff for lint and format, and a `py.typed` marker on any package whose types consumers should see. Use a `src/` layout — it stops tests from importing the working directory instead of the installed package, which is how a broken package still passes its own suite.

## Tooling baseline

`strict = true` covers most of it; these add the checks that catch real bugs rather than style. `--strict-markers` matters more than it looks: without it a typo'd `@pytest.mark.integraton` silently does nothing and the test runs where you thought it was excluded.

```toml
[tool.mypy]
strict = true
warn_unreachable = true
warn_redundant_casts = true
warn_unused_ignores = true

[[tool.mypy.overrides]]
module = "untyped_dep.*"
ignore_missing_imports = true

[tool.pytest.ini_options]
addopts = ["-ra", "--strict-markers", "--strict-config"]

conventions

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Conventions