Alexander Junge's website

Most projects I am working on enforce specific code styles, minimal requirements for documentation, or other rules every code contribution needs to follow. Git pre-commit hooks are very useful to automatically check that commits fulfill these requirements. This allows all project contributors to focus on the code instead of manually running different code formatters or wasting time in unnecessary arguments over missing whitespace.

Installing the pre-commit package manager

Pre-commit hooks are managed via the pre-commit package manager which can be installed from PyPI:

pip install pre-commit

Alternative installation methods are described on the project’s website: https://pre-commit.com/#install

Setting up pre-commit hooks

I frequently use pre-commits for the black code formatter and mypy static type checker. As an example, I will show how these tools can be installed as pre-commit hooks.

Create and open a file named .pre-commit-config.yaml in your local git repository where you specify the black and mypy hooks:

repos:
  - repo: https://github.com/psf/black
    rev: stable
    hooks:
      - id: black
        language_version: python3.7 # adjust to your version

  - repo: https://github.com/pre-commit/mirrors-mypy
    rev: 'eef73e2'  # Use the sha / tag you want to point at
    hooks:
      - id: mypy

Finally install the hooks by executing:

pre-commit install

Every time you run git commit both black and mypy will now be run against the files changed in the commit. More hooks are available at: https://pre-commit.com/hooks.html

Skipping git pre-commit hooks

Git allows you to skip all pre-commit hooks by adding the -n/--no-verify flag to git commit (note: this will also skip any commit-msg hooks). This is okay to do when you are in a hurry since any violations against the project’s rules should be called by downstream CI/CD checks anyway. But still: Don’t overuse this.