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

Rule tier reference

Per-file ignores

Relax rules where they create noise rather than value:

[tool.ruff.lint.per-file-ignores]
"tests/**/*.py" = ["S101", "D", "ANN", "PLR2004"]
"__init__.py" = ["F401"]   # Allow unused imports (re-exports)

Ruff Configuration

[tool.ruff]
target-version = "py310"    # Match your requires-python lower bound
line-length = 88            # Black default, de facto standard
src = ["src"]               # Critical for correct first-party import detection

[tool.ruff.lint.isort]
known-first-party = ["my_package"]

[tool.ruff.format]
docstring-code-format = true

Set src = ["src"] when using src layout. Without it, Ruff cannot distinguish first-party from third-party imports and isort grouping breaks silently.

Formatting with Ruff Format

Use ruff format as a drop-in Black replacement. Same defaults (line length 88, double quotes, magic trailing comma respected), but 30-100x faster.

Type Checking: mypy vs pyright

For library authors: Run both. Different checkers catch different issues. Pydantic, FastAPI, and httpx do this.

mypy strict configuration

[tool.mypy]
python_version = "3.10"
strict = true
warn_return_any = true
warn_unused_configs = true
enable_error_code = ["ignore-without-code", "redundant-cast", "truthy-bool"]

[[tool.mypy.overrides]]
module = "tests.*"
disallow_untyped_defs = false

Always specify error codes in # type: ignore[error-code]. Bare # type: ignore silences all errors on the line, hiding real bugs. Ruff rule PGH003 catches this automatically.

Modern Type Hints

Use the newest syntax your minimum Python version supports. Ruff's UP rules auto-fix old patterns.

Ship py.typed

Every typed package must include an empty py.typed marker in the package root. Without it, type checkers ignore your annotations for downstream users -- the most common typing mistake in the ecosystem.

src/my_package/
    __init__.py
    py.typed        # Empty file -- its presence signals typed package
    core.py

Add the PyPI classifier: "Typing :: Typed".

TYPE_CHECKING guard

Move type-only imports behind TYPE_CHECKING to avoid circular imports and reduce import time:

from __future__ import annotations
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from heavy_module import ExpensiveClass

Ruff's TCH rules detect imports that should be behind this guard.

Pre-commit Hooks

Collapse the entire pre-commit config to two Ruff hooks plus basic file hygiene. Do not add mypy, pytest, or security scanners -- they are too slow for pre-commit and belong in CI.

repos:
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v5.0.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-toml
      - id: check-added-large-files
        args: ["--maxkb=500"]
      - id: check-merge-conflict
      - id: debug-statements

  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.9.7
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

Run ruff (linter with --fix) before ruff-format so auto-fixes are properly formatted.

Reference Configuration

Complete pyproject.toml quality tooling section:

# -- Ruff ---------------------------------------------------------
[tool.ruff]
target-version = "py310"
line-length = 88
src = ["src"]

[tool.ruff.lint]
select = ["F", "E", "W", "I", "N", "UP", "B", "SIM", "C4", "RUF", "PERF", "TCH", "C90"]
ignore = ["E501"]

[tool.ruff.lint.per-file-ignores]
"tests/**/*.py" = ["S101", "PLR2004"]
"__init__.py" = ["F401"]

[tool.ruff.lint.isort]
known-first-party = ["my_package"]

[tool.ruff.lint.mccabe]
max-complexity = 10

[tool.ruff.format]
docstring-code-format = true

# -- mypy ----------------------------------------------------------
[tool.mypy]
python_version = "3.10"
strict = true
warn_return_any = true
warn_unused_configs = true
enable_error_code = ["ignore-without-code", "redundant-cast", "truthy-bool"]

[[tool.mypy.overrides]]
module = "tests.*"
disallow_untyped_defs = false

# -- Dependency group ----------------------------------------------
[dependency-groups]
lint = ["ruff>=0.9", "mypy>=1.14"]

Review Checklist

When reviewing code for quality tooling and type safety:

• Ruff is the sole linter and formatter -- no Black, isort, flake8, or their plugins installed separately
• tool.ruff.lint.select uses explicit opt-in, not select = ["ALL"] with a long ignore list
• src = ["src"] is set in [tool.ruff] for src-layout projects
• E501 is in the ignore list when using ruff format (formatter handles line length)
• mypy runs in strict mode (strict = true) with enable_error_code = ["ignore-without-code"]
• No bare # type: ignore comments -- all have specific error codes
• py.typed marker file exists in the package root and is included in the wheel
• Type hints use modern syntax appropriate for the minimum Python version (list[str], str | None)
• Pre-commit config contains only Ruff hooks and file hygiene -- no mypy, pytest, or security scanners
• ruff hook runs before ruff-format in pre-commit ordering
• Type checkers run in CI, not in pre-commit
• per-file-ignores relaxes rules for tests and __init__.py re-exports
• McCabe complexity threshold is set (max-complexity = 10) if C90 is enabled