Python Code Quality

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: python topic: quality for complete documentation.

Tool Stack (2026)

Tool	Purpose	Speed	Recommendation
ruff	Linting + Formatting	Fastest (10-100x)	Primary
mypy	Type checking	Good	CI gate
pyright	Type checking	Fast	IDE (Pylance)
ty	Type checking	Fastest	Emerging

Recommended setup: ruff (lint/format) + pyright (IDE) + mypy (CI)

ruff Configuration

# pyproject.toml
[tool.ruff]
line-length = 88
target-version = "py312"
src = ["src", "tests"]

[tool.ruff.lint]
select = [
    "E",      # pycodestyle errors
    "W",      # pycodestyle warnings
    "F",      # Pyflakes
    "I",      # isort
    "B",      # flake8-bugbear
    "C4",     # flake8-comprehensions
    "UP",     # pyupgrade
    "ARG",    # flake8-unused-arguments
    "SIM",    # flake8-simplify
    "TCH",    # flake8-type-checking
    "S",      # flake8-bandit (security)
    "RUF",    # Ruff-specific rules
]
ignore = [
    "E501",   # line too long (formatter handles)
    "S101",   # assert usage (OK in tests)
]

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

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

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

ruff Commands

# Lint
ruff check .
ruff check --fix .          # Auto-fix

# Format
ruff format .
ruff format --check .       # Check only

# Combined (CI)
ruff check . && ruff format --check .

mypy Configuration

# pyproject.toml
[tool.mypy]
python_version = "3.12"
strict = true
warn_return_any = true
warn_unused_ignores = true
disallow_untyped_defs = true
disallow_incomplete_defs = true
check_untyped_defs = true
no_implicit_optional = true
show_error_codes = true
enable_error_code = ["ignore-without-code", "truthy-bool"]

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

[[tool.mypy.overrides]]
module = ["third_party_lib.*"]
ignore_missing_imports = true

mypy Commands

# Check
mypy src/
mypy src/ --strict

# Daemon mode (faster)
dmypy run -- src/

# Generate stubs
stubgen -p my_package -o stubs/

pyright Configuration

// pyrightconfig.json
{
  "include": ["src"],
  "exclude": ["**/node_modules", "**/__pycache__"],
  "typeCheckingMode": "strict",
  "pythonVersion": "3.12",
  "reportMissingImports": true,
  "reportMissingTypeStubs": false,
  "reportUnusedImport": true,
  "reportUnusedVariable": true
}

Or in pyproject.toml:

[tool.pyright]
include = ["src"]
typeCheckingMode = "strict"
pythonVersion = "3.12"

Common Type Errors & Solutions

Missing return type

# Error: Function is missing a return type annotation
def greet(name: str):  # Bad
    return f"Hello {name}"

def greet(name: str) -> str:  # Good
    return f"Hello {name}"

Optional handling

# Error: "None" is not compatible with "str"
def get_name(user: User | None) -> str:
    return user.name  # Bad - user might be None

def get_name(user: User | None) -> str:
    if user is None:
        return "Unknown"
    return user.name  # Good - narrowed to User

Type narrowing

from typing import TypeGuard

def is_string_list(val: list[object]) -> TypeGuard[list[str]]:
    return all(isinstance(x, str) for x in val)

def process(items: list[object]) -> None:
    if is_string_list(items):
        # items is now list[str]
        print(items[0].upper())

Callable types

from collections.abc import Callable

# Function type
Handler = Callable[[str, int], bool]

def process(handler: Handler) -> None:
    result = handler("test", 42)

Generic variance

from typing import TypeVar

# Invariant (default)
T = TypeVar("T")

# Covariant (read-only)
T_co = TypeVar("T_co", covariant=True)

# Contravariant (write-only)
T_contra = TypeVar("T_contra", contravariant=True)

Strict Mode Migration

Phase 1: Basic types

[tool.mypy]
check_untyped_defs = true

Phase 2: Require annotations

[tool.mypy]
disallow_untyped_defs = true
disallow_incomplete_defs = true

Phase 3: Full strict

[tool.mypy]
strict = true

Per-module migration

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

CI/CD Integration

# GitHub Actions
name: Quality

on: [push, pull_request]

jobs:
  quality:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - name: Install uv
        uses: astral-sh/setup-uv@v4

      - name: Install dependencies
        run: uv sync

      - name: Lint
        run: uv run ruff check .

      - name: Format check
        run: uv run ruff format --check .

      - name: Type check
        run: uv run mypy src/

Pre-commit Configuration

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.14.0
    hooks:
      - id: ruff
        args: [--fix]
      - id: ruff-format

  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v1.14.0
    hooks:
      - id: mypy
        additional_dependencies: [types-requests]

# Install hooks
pre-commit install

# Run manually
pre-commit run --all-files

Type Stubs

# Install type stubs
uv add --dev types-requests types-redis

# Common stubs packages
# types-requests, types-redis, types-PyYAML
# types-python-dateutil, types-setuptools

Quality Metrics

Metric	Target	Tool
ruff violations	0	ruff check
mypy errors	0	mypy --strict
Type coverage	100%	mypy reports
Cyclomatic complexity	<10	ruff --select=C901

Anti-Patterns

Anti-Pattern	Why It's Bad	Solution
# type: ignore without code	Silences all errors	Use # type: ignore[error-code]
Any everywhere	No type safety	Use proper types or generics
Ignoring mypy in CI	Regressions slip in	Make it a required check
Not using --strict	Missing errors	Enable progressively
Manual formatting	Inconsistent, slow	Use ruff format

Quick Troubleshooting

Issue	Cause	Solution
"Module has no attribute"	Missing stubs	Install types-* package
"Incompatible types"	Type mismatch	Check expected vs actual type
"Cannot infer type"	Complex expression	Add explicit annotation
"Unused ignore"	Fixed error	Remove the ignore comment
ruff/mypy disagree	Different rules	Configure both consistently