Pre-Commit Tip

January 31, 2020

pre-commit is a straightforward way to ensure basic checks are maintained in a respository, before errors are even committed to a local branch.

This is particularly useful for light-touch validations like linting.

Find out here how to:

Install pre-commit locally
Configure pre-commit to lint all your files on commit
Use GitHub actions to ensure nothing is merged pre-commit

Installing

As a user, you need to ensure you have the pre-commit installed first:

brew install pre-commit

Each repo needs it installed separately. From a checked out repository, install by running:

pre-commit install

This will now run pre-commit each time you git commit. If you want to run pre-commit manually without committing, run:

pre-commit run --all-files

Configuring

On it’s own it won’t actually do anything. You need to provide a .pre-commit-config.yaml in the repository root, something like:

repos:
  - repo: https://github.com/pre-commit/pre-commit-hooks
    rev: v2.4.0
    hooks:
      - id: trailing-whitespace
      - id: end-of-file-fixer
      - id: check-yaml
      - id: check-merge-conflict

There are lots of other hooks, see the supported list here: pre-commit.com/hooks.html

Caveat

Installing pre-commit in a repo on your machine won’t automatically make pre-commit work in that repo on your colleagues machine.

Remember that pre-commit install is making a change to your local revision.

Work Around

Make use of GitHub actions, to run pre-commit checks post-push!

Add the following to .github/workflows/pre-commit.yml (source github.com/pre-commit/action)

name: pre-commit

on:
  pull_request:
  push:
    branches: [master]

jobs:
  pre-commit:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v1
      - uses: actions/setup-python@v1
      - name: set PY
        run: echo "::set-env name=PY::$(python --version --version | sha256sum | cut -d' ' -f1)"
      - uses: actions/cache@v1
        with:
          path: ~/.cache/pre-commit
          key: pre-commit|${{ env.PY }}|${{ hashFiles('.pre-commit-config.yaml') }}
      - uses: pre-commit/action@v1.0.1

This way if someone checks in badly formatted YAML because they forgot to pre-commit install, this workflow will catch them in the act before it is merged into master.