python-dev

What this skill does

# Python Development Setup

Opinionated, production-ready Python development stack. No choices to make - just use this.

## When to Use

- Starting a new Python project
- Modernizing an existing project (migrating from pip/poetry/mypy/black/flake8)
- Setting up linting, formatting, type checking, or testing
- Creating a Justfile for project commands
- Configuring pyproject.toml as the single source of truth

## The Stack

| Tool | Role | Replaces |
|------|------|----------|
| [uv](https://docs.astral.sh/uv/) 0.11+ | Package manager, Python versions, runner | pip, poetry, pyenv, virtualenv |
| [ty](https://docs.astral.sh/ty/) (beta) | Type checker (Astral, Rust) | mypy, pyright |
| [ruff](https://docs.astral.sh/ruff/) | Linter + formatter | flake8, black, isort, pyupgrade |
| [pytest](https://docs.pytest.org/) | Testing | unittest |
| [just](https://just.systems/) | Command runner | make |

> **Note on ty**: ty is in beta (0.0.x). It's fast and improving rapidly, but still missing features and may produce false positives on heavy-typing libraries (Pydantic, Django, SQLAlchemy). For projects that need rock-solid type checking today, swap `ty` for `pyright` and keep the rest of the stack unchanged.

## Quick Start: New Project

```bash
# 1. Create project with src layout
uv init --package my-project
cd my-project

# 2. Pin Python version
uv python pin 3.13

# 3. Add dev dependencies
uv add --dev ruff ty pytest pytest-asyncio pre-commit

# 4. Create Justfile (see template below)
# 5. Configure pyproject.toml (see template below)
# 6. Run checks
just check
```

## pyproject.toml Template

This is the single config file. Copy this and adjust `[project]` fields.

```toml
[project]
name = "my-project"
version = "0.1.0"
description = "Project description"
readme = "README.md"
requires-python = ">=3.13"
license = {text = "MIT"}
dependencies = []

[project.scripts]
my-project = "my_project:main"   # CLI: `uv run my-project` -> main() in src/my_project/__init__.py

[dependency-groups]
dev = [
    "ruff>=0.15.0",
    "ty>=0.0.30",
    "pytest>=9.0.0",
    "pytest-asyncio>=1.3.0",
    "pre-commit>=4.0.0",
]

[build-system]
requires = ["hatchling"]
build-backend = "hatchling.build"

[tool.hatch.build.targets.wheel]
packages = ["src/my_project"]

# =============================================================================
# RUFF - Loose, helpful rules only
# =============================================================================
[tool.ruff]
target-version = "py313"
line-length = 100

[tool.ruff.lint]
select = [
    "E",   # pycodestyle errors - syntax issues
    "F",   # pyflakes - undefined vars, unused imports
    "I",   # isort - import sorting
    "UP",  # pyupgrade - modern Python syntax
]
ignore = [
    "E501",   # line too long - formatter handles it
    "E741",   # ambiguous variable name - sometimes makes sense
    "UP007",  # X | Y unions - Optional[X] is more readable
]
exclude = [".git", ".venv", "__pycache__", "build", "dist"]

[tool.ruff.format]
quote-style = "double"
indent-style = "space"
line-ending = "lf"

# =============================================================================
# TY - Type Checker
# =============================================================================
[tool.ty.environment]
python-version = "3.13"

[tool.ty.src]
include = ["src"]

# =============================================================================
# PYTEST
# =============================================================================
[tool.pytest.ini_options]
testpaths = ["tests"]
python_files = ["test_*.py"]
python_classes = ["Test*"]
python_functions = ["test_*"]
asyncio_mode = "auto"
addopts = [
    "--strict-markers",
    "--strict-config",
    "-ra",
]
```

## Justfile Template

```just
# Check types and lint
check:
    uv run ty check
    uv run ruff check --fix && uv run ruff format

# Run tests
test *ARGS:
    uv run pytest {{ARGS}}

# Run tests with coverage
test-cov:
    uv run pytest --cov=src --cov-report=term-missing

# Auto-fix and format
fix:
    uv run ruff check --fix
    uv run ruff format

# Install/sync all dependencies
install:
    uv sync --all-groups

# Update all dependencies
update:
    uv lock --upgrade
    uv sync --all-groups

# Clean build artifacts
clean:
    rm -rf dist/ build/ .pytest_cache/ .ruff_cache/ htmlcov/
    find . -type d -name __pycache__ -exec rm -rf {} + 2>/dev/null || true
```

## Pre-commit Config

Create `.pre-commit-config.yaml`:

```yaml
repos:
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v6.0.0
    hooks:
      - id: check-added-large-files
        args: ['--maxkb=1000']
        exclude: ^uv\.lock$
      - id: detect-private-key
      - id: check-merge-conflict
      - id: check-yaml
      - id: check-toml
      - id: end-of-file-fixer
      - id: trailing-whitespace
      - id: mixed-line-ending
        args: ['--fix=lf']

  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.15.12
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

exclude: |
  (?x)^(
    \.venv/.*|
    \.git/.*|
    __pycache__/.*|
    build/.*|
    dist/.*|
    uv\.lock
  )$
```

Then run: `uv run pre-commit install`

## Project Structure

Always use src layout:

```
my-project/
  src/
    my_project/
      __init__.py
      cli.py
      models.py
  tests/
    conftest.py
    test_models.py
  pyproject.toml
  Justfile
  uv.lock
  .python-version
  .pre-commit-config.yaml
  .gitignore
```

## Daily Workflow

```bash
just check          # Type check + lint + format
just test           # Run tests
just test -x        # Stop on first failure
just fix            # Auto-fix lint issues
uv add httpx        # Add a dependency
uv add --dev hypothesis  # Add dev dependency
uv sync             # main deps + dev (dev is in default-groups)
uv sync --all-groups  # everything in [dependency-groups]
uv run python -m my_project  # Run the project
```

## CI (GitHub Actions)

Mirror `just check` + `just test` in CI. Drop this in `.github/workflows/ci.yml`:

```yaml
name: CI
on: [push, pull_request]

jobs:
  check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: astral-sh/setup-uv@v6
        with:
          enable-cache: true
      - run: uv sync --all-groups
      - run: uv run ty check
      - run: uv run ruff check --output-format github
      - run: uv run ruff format --check
      - run: uv run pytest
```

`astral-sh/setup-uv` installs uv, manages the Python install requested by `.python-version`, and caches the resolver. No separate `setup-python` step needed.

## Existing Project Migration

```bash
# 1. Install uv if not present
brew install uv

# 2. Convert requirements.txt to pyproject.toml deps
uv add -r requirements.txt

# 3. Replace mypy with ty
uv remove --dev mypy
uv add --dev ty

# 4. Replace black/flake8/isort with ruff
uv remove --dev black flake8 isort
uv add --dev ruff

# 5. Apply pyproject.toml config sections from template above
# 6. Create Justfile from template above
# 7. Run: just check
```

## Reference Docs

Detailed guides for each tool in `references/`:
- **uv-reference.md** - Project init, dependencies, lock/sync, Python versions, build/publish
- **ty-reference.md** - Configuration, rules, CLI flags, known limitations
- **ruff-reference.md** - Rule sets, formatter options, per-file ignores, CI integration
- **pytest-reference.md** - Plugins, fixtures, async testing, conftest patterns
- **justfile-reference.md** - Syntax, variables, parameters, shebang recipes, settings

## Resources

- [uv docs](https://docs.astral.sh/uv/) | [uv GitHub](https://github.com/astral-sh/uv)
- [ty docs](https://docs.astral.sh/ty/) | [ty GitHub](https://github.com/astral-sh/ty)
- [ruff docs](https://docs.astral.sh/ruff/) | [ruff GitHub](https://github.com/astral-sh/ruff)
- [pytest docs](https://docs.pytest.org/en/stable/)
- [just manual](https://just.systems/man/en/)