Pre-Commit Hooks

What is a pre-commit hook?

A small script that runs automatically before every git commit.

• Catches problems early

  Before code ever reaches GitHub — on your machine, every time.

• Shared across the team

  The config lives in the repo. Everyone gets the same checks automatically.

• Can fix things for you

  Many hooks (like formatters) just fix the issue silently. Others block the commit and tell you what's wrong.

• Can't be skipped by accident

  If a hook fails, the commit doesn't happen. The bar is raised for everyone.

---

How does it work?

The flow

1. You run git commit
2. Pre-commit intercepts it
3. Each hook runs in order
4. All pass → commit goes through
5. Any fail → commit is blocked, fix and retry

The config file

One file in your repo controls everything:

.pre-commit-config.yaml

repos:
  - repo: https://github.com/astral-sh/ruff-pre-commit
    rev: v0.4.0
    hooks:
      - id: ruff         # linting
      - id: ruff-format  # formatting

• Hooks are pulled from public repos
• Pin the version with rev:
• Run pre-commit install once to activate

---

Hooks we'll use today

• gitleaks — blocks accidentally committed passwords or API keys
• ruff — automatically fixes Python linting violations
• ruff-format — automatically formats Python code
• nbstripout — strips outputs from Jupyter notebooks before committing