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.

Rule tier reference

Tier	Prefixes	Notes
Must have	F, E, W, I, UP, B	Every top package enables these. Zero noise.
Recommended	N, SIM, C4, RUF, PERF	Enabled by Pydantic, Polars, attrs. Catches real issues.
Consider	S, D, TCH, PT, C90, PLR	Useful with targeted ignores. TCH valuable for typed libraries.
Optional	A, FBT, ARG, ERA, ANN	Opinionated or high false-positive rate. Cherry-pick individual rules.

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.

Setting	Keep Default	Change Only If
quote-style = "double"	Yes	Team strongly prefers single quotes
line-length = 88	Yes	Corporate standard requires different
skip-magic-trailing-comma = false	Yes	Never change -- preserves developer formatting intent
docstring-code-format = true	Enable this	No reason not to

Type Checking: mypy vs pyright

Aspect	mypy	pyright
Use when	Need plugins (pydantic, django, sqlalchemy)	Want fastest checks, strictest analysis
Strictness	strict = true flag	typeCheckingMode = "strict"
Speed	Moderate (use daemon mode)	Consistently fast
IDE	External tool	Powers VS Code Pylance
Plugin support	Rich ecosystem	Limited

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.

pyright configuration (for libraries running both checkers)

[tool.pyright]
pythonVersion = "3.10"
typeCheckingMode = "standard"
reportUnnecessaryTypeIgnoreComment = true
enableTypeIgnoreComments = false

typeCheckingMode = "standard" is the practical starting point -- strict pyright is significantly noisier than strict mypy, and most exemplar libraries (Pydantic, Flask, SQLAlchemy) do not use strict mode. enableTypeIgnoreComments = false separates suppression concerns: # type: ignore[code] is for mypy only, # pyright: ignore[rule] is for pyright only. This is the pattern typeshed uses and eliminates cross-contamination between the two checkers.

Modern Type Hints

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

Old Pattern	Modern Pattern	Since
List[str], Dict[str, Any]	list[str], dict[str, Any]	Python 3.9
Optional[str], Union[str, int]	str | None, str | int	Python 3.10
TypeVar("T") + Generic[T]	class Foo[T]:	Python 3.12 (PEP 695)
TypeAlias = Union[...]	type Alias = str | int	Python 3.12 (PEP 695)
from __future__ import annotations	Remove it	Python 3.14 (PEP 649)

Ship py.typed

Every typed package must include an empty py.typed marker file and the Typing :: Typed classifier. Without it, type checkers ignore your annotations for downstream users. See the project-structure skill for the full explanation, file placement, and PEP 561 details.

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.

Pydantic and runtime annotation frameworks: Do not guard imports behind TYPE_CHECKING if the type is used in Pydantic models, dataclasses with runtime validation, or any framework that evaluates annotations at runtime (serializers, dependency injection containers). Pydantic needs the actual class at runtime for validation -- guarding it behind TYPE_CHECKING causes NameError. When using TCH rules, configure Ruff to recognize your runtime annotation framework: [tool.ruff.lint.flake8-type-checking] runtime-evaluated-base-classes = ["pydantic.BaseModel"].

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

# -- pyright (for libraries running both checkers) -----------------
[tool.pyright]
pythonVersion = "3.10"
typeCheckingMode = "standard"
reportUnnecessaryTypeIgnoreComment = true
enableTypeIgnoreComments = false

# -- Dependency group ----------------------------------------------
[dependency-groups]
lint = ["ruff>=0.9", "mypy>=1.14", "pyright>=1.1"]

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
• For library projects, pyright is configured alongside mypy (typeCheckingMode = "standard", enableTypeIgnoreComments = false)
• 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
• TYPE_CHECKING guards do not hide types used by runtime validation frameworks (Pydantic, attrs, dataclasses)