python-standards skill

---
name: python-standards
description: |
  Modern Python development with uv, ruff, mypy, and pytest. Use when:
  - Writing or reviewing Python code
  - Setting up Python projects or pyproject.toml
  - Choosing dependency management (uv, poetry, pip)
  - Configuring linting, formatting, or type checking
  - Organizing Python packages
  Keywords: Python, pyproject.toml, uv, ruff, mypy, pytest, type hints,
  virtual environment, lockfile, package structure
effort: high
---

# 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 |

```bash
# 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:**
```python
def fetch_user(user_id: str) -> User | None:
    """Fetch user by ID."""
    ...

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

**mypy strict mode:**
```toml
[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:**
```python
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):
```toml
[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](references/testing-patterns.md) - pytest, fixtures, parametrize
- [ruff-config.md](references/ruff-config.md) - Complete ruff rule configuration