Matt Canty's Writing Place

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


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


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: 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:


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

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.

I am Matt Canty, an engineer of varying titles who helps tech companies run reliable and fault-tolerant software platforms. Just send me an email:

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