code-quality

Use Ruff as the Single Linter and Formatter, Enforce Types with mypy

The modern Python code quality stack has consolidated around two tools: Ruff for linting and formatting, mypy (or pyright) for type checking. Ruff replaces Black, isort, flake8, pyupgrade, autoflake, and all flake8 plugins with a single Rust-powered binary that is 10-100x faster. Every major package -- FastAPI, Pydantic, httpx, Polars, Rich, pytest, attrs -- has migrated to Ruff. There is no credible alternative for new projects.

Without this consolidation, teams juggle conflicting configurations across six or more tools, deal with parser incompatibilities, and waste CI minutes. Without type checking in CI, a py.typed marker becomes a lie -- promising type safety to downstream consumers while delivering none.

Ruff Rule Sets

Start with an explicit select of rules you want. Never use select = ["ALL"] with a long ignore list -- every Ruff update adds new rules that fire unexpectedly.

Recommended starting set

[tool.ruff.lint]
select = [
    "F",      # Pyflakes -- undefined names, unused imports
    "E",      # pycodestyle errors
    "W",      # pycodestyle warnings
    "I",      # isort -- import sorting
    "N",      # pep8-naming -- naming conventions
    "UP",     # pyupgrade -- modernize syntax
    "B",      # flake8-bugbear -- common bugs
    "SIM",    # flake8-simplify -- simplifiable code
    "C4",     # flake8-comprehensions -- better comprehensions
    "RUF",    # Ruff-specific rules
    "PERF",   # Perflint -- performance anti-patterns
]
ignore = ["E501"]  # Line too long -- handled by formatter

Also consider adding:

• "TCH" -- moves type-only imports behind if TYPE_CHECKING: guards, reducing import time and breaking circular imports. Most valuable for typed libraries where downstream users pay your import cost.
• "C90" -- flags functions exceeding a cyclomatic complexity threshold (default 10). A code review guardrail for functions that are too complex to maintain and test. Pair with [tool.ruff.lint.mccabe] max-complexity = 10.