Python Standards

Modern Python with uv + ruff + mypy + pytest. All config in pyproject.toml.

Toolchain

Tool Purpose

uv Dependencies + virtual envs (lockfile: uv.lock )

ruff Linting + formatting (replaces black, isort, flake8)

mypy Type checking (strict mode)

pytest Testing + coverage

Setup

uv init && uv add <deps> && uv sync

Daily workflow

uv run ruff check . --fix && uv run ruff format . uv run mypy . && uv run pytest

Type Hints

All functions and classes must have explicit type hints:

def fetch_user(user_id: str) -> User | None: """Fetch user by ID.""" ...

def process_items(items: list[Item]) -> dict[str, int]: ...

mypy strict mode:

[tool.mypy] strict = true disallow_untyped_defs = true disallow_any_generics = true

Package Structure

Feature-based, not layer-based:

src/myproject/ users/ # Domain init.py # Public API: all = ['User', 'UserService'] models.py services.py orders/ # Another domain shared/ # Cross-cutting concerns

Error Handling

Catch specific exceptions with context:

try: result = parse_config(path) except FileNotFoundError: logger.warning(f"Config not found: {path}") return defaults except json.JSONDecodeError as e: raise ConfigError(f"Invalid JSON in {path}") from e

Never: bare except: , silent swallowing, except Exception .

pyproject.toml

Single source of truth (no setup.py, requirements.txt):

[project] name = "myproject" version = "1.0.0" requires-python = ">=3.11" dependencies = ["httpx", "pydantic"]

[project.optional-dependencies] dev = ["pytest", "mypy", "ruff"]

[tool.ruff] select = ["E", "W", "F", "I", "B", "UP", "SIM", "S", "ANN"] line-length = 88

[tool.pytest.ini_options] testpaths = ["tests"] addopts = ["--cov=src", "--cov-fail-under=85"]

Anti-Patterns

• Any without documented justification

• Layer-based folders (/controllers , /models , /views )

• Circular imports

• Legacy tools (pip, black+isort, flake8)

• Multiple config files

• noqa comments without justification

References

• testing-patterns.md - pytest, fixtures, parametrize

• ruff-config.md - Complete ruff rule configuration