python

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:

  _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.

[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"]