python-best-practiceslisted

# Python best practices The tools decide most of this. `ruff` settles formatting, imports, line length and lint rules; `mypy` settles types. Run them rather than reasoning about them: ``` make fast formatting, lint, types — the edit loop make fix apply safe autofixes first, then check make quality the full gate, including tests and coverage ``` **Never satisfy a check by disabling it.** A blanket `# noqa`, a widened `ignore` list or a deleted assertion converts a real signal into a false one. If a rule genuinely does not fit this project, change the rule in `pyproject.toml` as a deliberate, justified decision — not inline, silently. What follows is the part tools cannot check. ## The standards that are not negotiable **Type hints on every signature**, parameters and return alike. Modern syntax, since this project targets Python 3.12: ```python def get_owned_category(db: Session, user: User, category_id: int) -> Category: ``` Use `X | None`, `list[X]`, `dict[str, X]`. Not `Optional[X]`, `List[X]`, `Dict[str, X]` — those are the pre-3.10 spelling and only add noise now. **78-character lines.** Break with parentheses, never backslashes. The formatter does most of it; you break the long strings and comments it cannot. **Google-style docstrings** on public functions, classes and modules. **Exception chaining.** Always `raise NewError(...) from err`. Dropping the cause discards the traceback that explains the failure: ```python try: user_id = decode_acc