GitHub Action to run mypy on changed files only

Recently, I needed a way to perform Python static type checking using mypy only on "changed" files for a GitHub Pull Request.

So I googled a bit and found out this super useful action that you can use it in your workflow right away.

And after customizing some options like filter only *.py files, I was able to run mypy passing the changed files as argument.

It's worth to mention that I needed to include --ignore-missing-imports argument to mypy because I'm not scanning the whole repo, only some files. The goal of course is to check them all. However, a lot of fixes should be made before it's ready.

This solution can be very useful to you if you want to include static type checking for new files on a legacy project where existing files will be fixed little by little.

name: "mypy check"
on: [pull_request]

jobs:

  static-type-check:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/checkout@v2
    - uses: actions/setup-python@v3
      with:
        python-version: '3.x'
    - run: pip install mypy  # you can pin your preferred version
    - name: Get Python changed files
      id: changed-py-files
      uses: tj-actions/changed-files@v23
      with:
        files: |
          *.py
          **/*.py
    - name: Run if any of the listed files above is changed
      if: steps.changed-py-files.outputs.any_changed == 'true'
      run: mypy ${{ steps.changed-py-files.outputs.all_changed_files }} --ignore-missing-imports

Happy type-safety!