# Python > Use when the task is Python itself, in any framework or none: PEP 695 generics, mypy --strict typing, dataclass/Protocol/TypedDict/Enum choices, asyncio.TaskGroup, stdlib idioms, src/ layout + pyproject.toml with uv, ruff+mypy+pytest gate. NOT a FastAPI/ASGI service (that is fastapi), NOT a deep pytest suite (that is testing-py). - Skill: `ericrisco/python` (Agent Skill, multi-file: 7 files) - Install (CLI): `npx skillmds@latest add ericrisco/python` - Raw SKILL.md: https://api.skillmd.com/api/skills/ericrisco/python/raw - Safety review: pending - Works with: Claude Code, Claude.ai, OpenAI Codex - Category: Productivity - Author: ericrisco (https://skillmd.com/u/ericrisco) - Updated: 2026-09-17 - Page: https://skillmd.com/skills/ericrisco/python --- # Modern Python at the language level Write, review, modernize, type, and package Python that reads like a typed, flat-control-flow, stdlib-first program whose dependencies and tooling all live in one `pyproject.toml`. Types are part of the design, not decoration; the stdlib is large and you reach for it before a dependency; correctness is `ruff` + a type checker + `pytest` in one gate. Targets **Python 3.12+ (floor) / 3.14 (current**, released 7 Oct 2025): PEP 695 inline type parameters (`class Box[T]:`, `type Alias = ...`), `asyncio.TaskGroup`, and — in 3.14 — deferred annotation evaluation by default (PEP 649/749, no more `from __future__ import annotations`), PEP 750 template strings (`t"..."`), and `compression.zstd`. Tooling pins: **uv 0.11** (project + package manager), **ruff 0.15** (lint + format), **mypy 1.20 `--strict`** (or Astral's `ty`, still preview — default to mypy), **pytest 8**. ## Scope — what this skill delegates - Building a FastAPI / ASGI service (routes, Pydantic models, SQLAlchemy, uvicorn) -> [`fastapi`](../fastapi/SKILL.md). That skill owns the service shape; this one owns the language it is written in. - A pytest suite as the primary task — fixtures, parametrize matrices, mocking, coverage, property-based testing -> `testing-py` (this skill keeps only the *baseline*: a few tests so `verify.sh` has something to run, then hands off). - Language-agnostic threat modeling / authz / OWASP review -> [`secure-coding`](../secure-coding/SKILL.md) (this skill keeps Python-specific safety: no `eval`/`pickle` of untrusted data, `subprocess` without `shell=True`, `secrets` over `random`). - Containerfile / CI pipeline / deploy mechanics -> [`deployment`](../deployment/SKILL.md) (this skill ships only a uv-based CI note). - Another language -> `go`, `typescript`, `rust`, etc. Django ORM/models/migrations -> `django`. Python typing, async language semantics, and uv packaging live **here**, not in a separate skill — this skill is the canonical authority for the language substrate under any Python program. ## Decision rules Apply on every Python edit: 1. **Type the boundary.** Every public function, method, and module-level name gets an annotation; let inference handle locals. Untyped public API defeats `--strict`. 2. **Flat control flow.** Guard-clause and `return`/`raise` early; keep the happy path unindented — arrow code hides the logic. 3. **Stdlib before a dependency.** `pathlib`, `itertools`, `functools`, `dataclasses`, `collections` cover most needs; a new dep is a maintenance liability you must justify. 4. **Prefer immutability.** `@dataclass(frozen=True, slots=True)` for value objects; mutate only where you must — shared mutable state is the bug you debug at 2am. 5. **Everything passes the gate.** `ruff check` + `ruff format --check` + `mypy --strict` + `pytest` — green locally via `scripts/verify.sh` before you call it done. ## Typing Type the boundary; run `mypy --strict` so untyped code and implicit `Any` are errors, not silent gaps. Use **PEP 695** inline syntax for all new generic code — no explicit `TypeVar` objects: ```python # Good (3.12+): inline type parameter and the `type` alias statement. def first[T](xs: list[T]) -> T: return xs[0] class Box[T]: ... type UserId = int # `type` statement: a real alias, lazily evaluated # Bad (legacy): `T = TypeVar("T")` then a Generic[T] — fine in old code, don't write it new. ``` Core choices: `Protocol` (structural, no inheritance) over an ABC when you only need "has these methods"; `X | None` (not `Optional[X]`); `Literal`/`Enum` for closed value sets; `Final` for constants; `Self` for fluent returns; `@overload` for signature families. Narrow with `isinstance`, `assert`, or an `is None` guard — mypy follows the flow. In **3.14** annotations are lazy by default (PEP 649/749), so forward references resolve without `from __future__ import annotations`. Full PEP 695 bounds/constraints/variance, `Protocol` vs ABC, `TypedDict` `Required`/`NotRequired`, `ParamSpec`/`TypeVarTuple`, `TypeGuard`/`TypeIs`, `cast`, and common `--strict` errors with fixes -> [`references/typing.md`](references/typing.md). ## Data modeling Pick the carrier by what the data *is*, not by habit: | Need | Use | Why | | --- | --- | --- | | Immutable value object, typed fields, methods | `@dataclass(frozen=True, slots=True)` | Hashable, no `__dict__` overhead, real types | | Small fixed tuple, positional + named, immutable | `NamedTuple` | Tuple semantics + field names; cheap | | Shape of an external/JSON dict, no runtime class | `TypedDict` | Types a plain `dict` without wrapping it | | Closed set of named constants | `Enum` / `StrEnum` / `IntEnum` | One source of truth; `Literal`-friendly | | Mutable bag of related state with behavior | plain class / `@dataclass` | When you genuinely need mutation | | Runtime-validated I/O model (parse untrusted data) | Pydantic -> [`fastapi`](../fastapi/SKILL.md) | Validation is a service concern, not language | ```python from dataclasses import dataclass @dataclass(frozen=True, slots=True) class Point: x: float y: float def translated(self, dx: float, dy: float) -> "Point": return Point(self.x + dx, self.y + dy) # returns a new value, never mutates ``` Frozen-slots dataclass when you want methods + hashability + clear types; `NamedTuple` when the thing genuinely *is* a small tuple you also unpack positionally. ## Stdlib idioms Reach into the stdlib before adding a dependency. - **`pathlib`** for all filesystem paths: `Path("data") / name`, `p.read_text()`, `p.glob("*.json")`, `p.with_suffix(".bak")` — typed and composable, never `os.path.join`. - **`collections`**: `defaultdict(list)`, `Counter(words)`, `deque(maxlen=100)` for ring buffers. - **`itertools`**: `chain`, `groupby`, `islice`, `batched` (3.12+) instead of hand-rolled loops. - **`functools`**: `@cache` / `@lru_cache` for pure memoization, `@cached_property`, `partial`. - **`contextlib`**: `@contextmanager`, `ExitStack` for dynamic resource sets, `suppress(FileNotFoundError)`. - **`logging`**, never `print` in a library: `print` writes to a caller's stdout you do not own; `logging.getLogger(__name__)` lets them configure it. Prefer comprehensions over `map`/`filter`+`lambda`; prefer a generator (`(... for ...)`) when you only iterate once. Use `match` for structural dispatch over a chain of `isinstance`: ```python from functools import cache from pathlib import Path @cache def config_dir() -> Path: # computed once, memoized return Path.home() / ".config" / "myapp" def area(shape: object) -> float: match shape: # structural dispatch, captures by attribute/key case {"kind": "circle", "r": float(r)}: return 3.14159 * r * r case _: raise TypeError(f"unknown shape: {shape!r}") ``` f-strings for formatting; note 3.14's PEP 750 `t"..."` template strings yield a `Template` (not a `str`) for *safe custom interpolation* (e.g. escaping) — use them when an f-string would inject untrusted text. Full cookbook (itertools/functools/collections recipes, `match` patterns, dataclass `field`/`default_factory`/`__post_init__`, `Enum`/`StrEnum`/`IntFlag`) -> [`references/stdlib.md`](references/stdlib.md). ## Errors & resources Define a small exception hierarchy rooted in one base so callers can catch broadly or narrowly; chain causes with `raise ... from`; never write a bare `except:`. ```python class AppError(Exception): ... class NotFoundError(AppError): ... def load(path: Path) -> str: try: return path.read_text() except FileNotFoundError as e: raise NotFoundError(f"missing {path}") from e # preserves the cause chain ``` Use `except*` to handle an `ExceptionGroup` (what a `TaskGroup` raises) by member type. Prefer **EAFP** (try the operation, handle the failure) over LBYL race-prone pre-checks. Always release resources with `with` (a context manager), not manual try/finally close. ## Async (language level) Use `asyncio.run(main())` as the single entry point. **`asyncio.TaskGroup` (3.11+) over bare `gather`** — it is the structured-concurrency primitive: a child failure cancels its siblings and surfaces as an `ExceptionGroup`, and no task outlives the block. ```python import asyncio # Bad: gather leaks the other tasks on first failure and loses structure. async def fetch_all_bad(ids: list[int]) -> list[bytes]: return await asyncio.gather(*(fetch(i) for i in ids)) # Good: TaskGroup — sibling cancellation on error, bounded lifetime, real grouping. async def fetch_all(ids: list[int]) -> list[bytes]: async with asyncio.TaskGroup() as tg: tasks = [tg.create_task(fetch(i)) for i in ids] return [t.result() for t in tasks] # block exited => all done or raised ``` Bound every wait with `async with asyncio.timeout(5.0):`. On `CancelledError`, clean up and **re-raise** — swallowing it breaks cancellation for the whole tree. Async is for **IO-bound** concurrency only; CPU-bound work blocks the loop — push it to `asyncio.to_thread` / a `ProcessPoolExecutor` (or 3.14's free-threaded build). **HTTP servers belong to [`fastapi`](../fastapi/SKILL.md), not here.** Runtime model, `ExceptionGroup`/`except*`, queues with backpressure, cancellation discipline, and sync<->async bridging -> [`references/async.md`](references/async.md). ## Project layout & packaging (uv) Use a `src/` layout so tests import the installed package, not the source tree by accident: ```text myapp/ src/myapp/__init__.py src/myapp/core.py tests/test_core.py pyproject.toml uv.lock # committed scripts/verify.sh ``` `pyproject.toml` is the single config — PEP 621 metadata, dependency groups, and tool config. Never hand-edit a `requirements.txt`; `uv add` writes the dep and updates `uv.lock`, which you commit. ```toml [project] name = "myapp" version = "0.1.0" requires-python = ">=3.12" dependencies = ["httpx>=0.27"] [project.scripts] myapp = "myapp.core:main" # console entry point [dependency-groups] dev = ["ruff>=0.15", "mypy>=1.13", "pytest>=8"] [build-system] requires = ["uv_build>=0.11"] build-backend = "uv_build" # uv's own backend, stable since July 2025 [tool.ruff] line-length = 100 [tool.mypy] strict = true ``` Core uv verbs (each updates `uv.lock`, which you commit): ```bash uv init --package myapp # scaffold pyproject.toml + src/ + .venv uv add httpx # add a runtime dep uv add --dev ruff mypy pytest # add to the dev group uv sync --frozen # install exactly from the lockfile (CI + fresh clones) uv run pytest -q # run inside the managed venv uv python install 3.14 # pin/install an interpreter ``` ## Quality gate + verify.sh One local gate, mirroring CI: `ruff check --fix .`, `ruff format .`, `mypy --strict src` (or `ty check`), `pytest -q`. `scripts/verify.sh` runs all of them, skips a missing tool/dir with a warning, and exits non-zero on any real failure — run `./scripts/verify.sh` from the project root before declaring done. CI is `astral-sh/setup-uv` + `uv sync --frozen` + the same four commands; full pipeline -> [`deployment`](../deployment/SKILL.md). ## pytest baseline Carry just enough to make `verify.sh` meaningful — plain `test_*` functions, `assert`, `pytest.raises`, one `parametrize`, `tmp_path` for files: ```python import pytest from myapp.core import head @pytest.mark.parametrize("xs, expected", [(["a", "b"], "a"), (["x"], "x")]) def test_head_returns_first(xs: list[str], expected: str) -> None: assert head(xs) == expected def test_head_rejects_empty() -> None: with pytest.raises(ValueError): head([]) def test_writes_file(tmp_path) -> None: (tmp_path / "f.txt").write_text("hi") assert (tmp_path / "f.txt").read_text() == "hi" ``` Deep fixtures, mocking, coverage gates, and property-based testing belong to `testing-py` — stop at the baseline and hand off. ## Security (embedded, Python-specific) Generic threat modeling and authz live in [`secure-coding`](../secure-coding/SKILL.md); these Python-specific controls stay here: ```python # Bad # Good eval(user_input) ast.literal_eval(user_input) # never eval/exec input pickle.loads(network_bytes) json.loads(network_bytes) # never unpickle untrusted data subprocess.run(cmd, shell=True) subprocess.run(["ls", path]) # list args, no shell=True random.random() # tokens secrets.token_urlsafe(32) # secrets, not random, for secrets ``` Keep deps locked (`uv.lock`) and audited (`pip-audit` / `uv` resolution); read secrets from env or a secret manager, never hardcode or log them. ## Anti-patterns -> STOP | Tempting move | Reality / do instead | | --- | --- | | "`def f(xs=[])` is fine, it's empty" | One list shared across all calls; use `= None` then `xs = xs or []`. | | "bare `except:` to be safe" | Swallows `KeyboardInterrupt`/bugs; catch a specific type. | | "`from module import *`" | Pollutes the namespace, breaks tooling; import names explicitly. | | "`print()` to debug this library" | Writes to a stdout you don't own; use `logging.getLogger(__name__)`. | | "`os.path.join` is what I know" | `pathlib.Path` is typed and composable; use `/`. | | "I'll add types later" | Untyped public API defeats `--strict`; type the boundary now. | | "edit `requirements.txt` by hand" | Drifts from the lock; `uv add` / `uv remove` and commit `uv.lock`. | | "`asyncio.gather` is simpler" | Leaks siblings on failure; `TaskGroup` for structured concurrency. | | "swallow `CancelledError`, it's noise" | Breaks cancellation for the whole tree; clean up and re-raise. | | "`time.sleep` inside this coroutine" | Blocks the event loop; `await asyncio.sleep(...)`. | | "`pickle.loads` the cache, it's ours" | Any untrusted byte = code execution; use `json`. | | "explicit `TypeVar` everywhere" | New code uses PEP 695 `def f[T]` / `class C[T]` / `type X`. | ## Project grounding (02-DOCS) In a project that has the [`harness`](../harness/SKILL.md) wiki, record this project's Python conventions in `02-DOCS/wiki/stack/python.md` and index it in `02-DOCS/wiki/index.md` — the interpreter floor, `src/` layout, uv workflow, ruff/mypy config, async-vs-sync stance, and data-modeling defaults. Read it first on every use and bump its `Updated` date when a convention changes. This is *recorded, not gated*: never block the task on it, and skip silently when there is no `02-DOCS/` layer.