Pre Commit

Pre-commit hooks are automated scripts that run before each Git commit, catching code quality issues, formatting problems, and security risks early before code reaches version control.

What Are Pre-commit Hooks?

• Automation: Run checks automatically on every commit
• Prevention: Stop bad code from entering repository
• Enforcement: Maintain standards across team
• Speed: Catch issues before code review (save time!)
• Consistency: Ensure everyone follows same rules

How Pre-commit Works

1. Developer runs `git commit`
2. Pre-commit hooks run on staged files
3. Hooks check formatting, lint, tests, security
4. If issues found, commit blocked, fixes suggested
5. Developer fixes and commits again

Installation & Setup

# Install pre-commit
pip install pre-commit

# Create .pre-commit-config.yaml
cat > .pre-commit-config.yaml << EOF
repos:
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.5.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-json
EOF

# Install hooks into git
pre-commit install

# Now hooks run automatically on every commit!

Popular Hook Categories

Python Code Quality:

repos:
  # Black formatter
  - repo: https://github.com/psf/black
    rev: 23.12.0
    hooks:
      - id: black
        language_version: python3

  # Ruff linter (fast, comprehensive)
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.1.9
    hooks:
      - id: ruff
        args: [--fix]

  # isort import sorting
  - repo: https://github.com/PyCQA/isort
    rev: 5.13.0
    hooks:
      - id: isort

  # mypy type checker
  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v1.7.1
    hooks:
      - id: mypy

JavaScript/Web:

# ESLint
  - repo: https://github.com/pre-commit/mirrors-eslint
    rev: v8.56.0
    hooks:
      - id: eslint
        types: [javascript]

  # Prettier formatter
  - repo: https://github.com/pre-commit/mirrors-prettier
    rev: v3.1.0
    hooks:
      - id: prettier
        types: [javascript, css, markdown]

Security & Secrets:

# Detect secrets
  - repo: https://github.com/Yelp/detect-secrets
    rev: v1.4.0
    hooks:
      - id: detect-secrets

  # Bandit for security issues
  - repo: https://github.com/PyCQA/bandit
    rev: 1.7.5
    hooks:
      - id: bandit

General/File Checks:

- repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.5.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-json
      - id: check-merge-conflict
      - id: check-docstring-first
      - id: debug-statements
      - id: mixed-line-ending
      - id: detect-private-key

Usage Examples

Basic Commands:

# Hooks run automatically on commit
git commit -m "Add feature"

# Skip hooks (not recommended!)
git commit --no-verify

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

# Run specific hook
pre-commit run black --all-files

# Update hooks
pre-commit autoupdate

# Clean up
pre-commit clean

Complete Configuration Example

# .pre-commit-config.yaml
repos:
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v4.5.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-json
      - id: mixed-line-ending

  - repo: https://github.com/psf/black
    rev: 23.12.0
    hooks:
      - id: black

  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.1.9
    hooks:
      - id: ruff
        args: [--fix]

  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: v1.7.1
    hooks:
      - id: mypy
        additional_dependencies: [types-all]

  - repo: https://github.com/Yelp/detect-secrets
    rev: v1.4.0
    hooks:
      - id: detect-secrets

Advantages

✅ Catch issues early: Before code review stage
✅ Consistent style: Automatic formatting enforcement
✅ Prevent secrets: Don't accidentally commit passwords
✅ Save time: Automate tedious checks
✅ Team alignment: Everyone uses same tools
✅ Faster reviews: PR reviewers focus on logic, not style
✅ Quality gates: Never commit broken code

Disadvantages

❌ Initial setup: Takes time to configure
❌ Performance: Can slow down commits if inefficient
❌ Learning curve: Teams must understand workflow
❌ Maintenance: Keep hooks updated with project needs

Best Practices

1. Keep hooks fast: <10 seconds total (use `--no-verify` sparingly)
2. Auto-fix when possible: Format, import sort automatically
3. Run on CI/CD too: Backup enforcement in pipeline
4. Document in README: Tell team why hooks exist
5. Update regularly: `pre-commit autoupdate` quarterly
6. Exclude files: Some files shouldn't be checked

exclude: ^(venv/|build/|migrations/)

7. Make skip easy for urgent fixes: Via documentation, not encouragement

Real-World Workflow

# Project setup
git clone project
cd project
pip install pre-commit
pre-commit install

# Now normal development
git add myfile.py
git commit -m "Add feature"

# Pre-commit runs automatically:
# - Black formats code
# - Ruff finds issues
# - mypy checks types
# - Detects secrets

# If issues found:
# Fix automatically (Black)
# Fix manually (Ruff warnings)
# git add fixed files
# git commit again (now passes!)

Comparison: Pre-commit vs CI/CD

Recommendation: Use both! Pre-commit for fast feedback, CI/CD for comprehensive validation.

Pre-commit hooks are the quality guardian for your repository — preventing bad code from entering version control while maintaining developer velocity through automation.