python-patterns

# Python Patterns This skill is for **day-to-day Python code quality**: readability, correctness, maintainability. If you are doing asyncio-heavy work, prefer `python-async-patterns`. If the task is profiling, GIL/free-threading, or experimental JIT, use `python-performance`. ## When to activate - Writing/refactoring Python modules, libraries, services - Reviewing PRs for idioms, clarity, and footguns - Introducing typing or improving error handling - Designing lightweight data models and APIs - Choosing threads vs processes vs subinterpreters (not asyncio) --- ## Core principles (high signal) - Readability beats cleverness. - Be explicit at boundaries (I/O, parsing, network). Keep inner code simple. - Prefer small, typed functions with clear names. - Use context managers for resource safety. - Raise specific exceptions and preserve causes (`raise ... from e`). --- ## Outcome expectations - Code is readable; intent is clear from names and structure, not comments. - All public APIs have type annotations; error boundaries are explicit. - Resource cleanup is deterministic (via context managers, no `__del__`). - Exceptions are domain-specific and chain causes; no silent failures. --- ## Recommended workflow 1. Design module interfaces with clear input/output types and error contracts. 2. Use context managers for all resource acquisition. 3. Validate at I/O boundaries; raise specific errors with cause chaining. 4. Apply ruff, mypy (on public APIs), and pytest baseline in CI. 5. Review code for footguns (mutable defaults, bare `except`, type hiding). --- ## Quick review checklist - No mutable default arguments; `None` sentinel used - `is None` / `is not None` (not `== None`) - Specific `except` clauses; no bare `except:` - Types: public functions/classes have annotations; 3.12+ type-parameter syntax; `T | None` not `Optional[T]` - Files/paths use `pathlib.Path` (`Path.walk` 3.12+); not `os.path` string soup - Iteration uses comprehensions/generators only when simple - CPU-bound fan-out uses executors from `concurrency.md`, not unbounded `Thread` spawns --- ## Resources Load on demand: - `references/typing.md` — load for annotations, PEP 695 generics, Protocols, 3.14 deferred evaluation - `references/errors.md` — load for exception hygiene, `ExceptionGroup` / `except*`, `finally` control flow - `references/data-models.md` — load for dataclasses, `copy.replace`, `StrEnum`, t-strings (3.14) - `references/iteration.md` — load for comprehensions, generators, `itertools.batched`, `match` - `references/resources.md` — load for context managers and pathlib - `references/concurrency.md` — load for threads, `concurrent.futures`, queues, multiprocessing start methods, `InterpreterPoolExecutor` - `references/layout-tooling.md` — load for `pyproject.toml`, ruff/mypy/pytest, removed stdlib modules - `references/performance.md` — one-screen "don't micro-optimize here"; real measurement is `python-performance`