Python Modern

When this skill is active, follow this 7-step discipline for Python projects:

1. Project Structure

Use the src layout with pyproject.toml as the single config file:

• src/<package>/ for source code, tests/ at the root
• pyproject.toml for all metadata, dependencies, and tool config — no setup.py, setup.cfg, or requirements.txt
• py.typed marker in the package root for PEP 561 type stub discovery
• .python-version file pinning the minimum supported version

2. Dependency Management with uv

Use uv for fast, reproducible dependency management:

• uv init for new projects, uv add <pkg> for dependencies
• uv add --dev pytest ruff mypy for dev dependencies
• uv lock to generate a lockfile, uv sync to install from it
• Pin Python version: uv python pin 3.12

3. Type Everything

Set up the project for strict typing from day one:

• Add from __future__ import annotations to every new file via a project template or snippet
• Run uv run mypy src/ --strict after adding each module to catch gaps immediately
• Audit untyped boundaries: any function accepting or returning dict or Any needs a typed replacement
• Generate stubs for untyped dependencies: uv run stubgen -p <package> or add [[tool.mypy.overrides]]

4. Configure Ruff

Set up ruff in pyproject.toml for linting and formatting:

[tool.ruff]
target-version = "py312"
line-length = 88

[tool.ruff.lint]
select = ["E", "F", "I", "N", "UP", "B", "SIM", "TCH", "RUF"]

• Run uv run ruff check --fix . before every commit
• Run uv run ruff format . for consistent formatting
• Fix all violations — no # noqa unless the rule is genuinely inapplicable

5. Configure Mypy

Set up mypy for strict type checking in pyproject.toml:

[tool.mypy]
strict = true
warn_return_any = true
warn_unreachable = true

• Run uv run mypy src/ — zero errors required
• For third-party packages without stubs, add specific [[tool.mypy.overrides]] instead of blanket ignores
• Never use bare # type: ignore — always include a specific error code: # type: ignore[override]

6. Write Tests with pytest

Structure tests to mirror the source layout:

• tests/test_<module>.py for each source module
• Use pytest fixtures for setup/teardown, conftest.py for shared fixtures
• Use pytest.mark.parametrize for multiple input cases
• Run with uv run pytest -x --tb=short — stop on first failure during development

7. Verify the Full Pipeline

Before marking work complete, run the full verification chain:

• uv run ruff check . — zero lint errors
• uv run ruff format --check . — formatting matches
• uv run mypy src/ — zero type errors
• uv run pytest — all tests pass
• If any step fails, fix and re-run the entire chain

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