Only run black on changed files - github-actions

Question

I'm using the official black GitHub Action. Currently, whenever I push changes, black runs on the entire repository. However, I only want it to run on the changed files. I've tried using some of the GitHub environment variables, but to no avail. Here's my workflow yaml:
name: Lint
on: [push, pull_request]
jobs:
lint:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout#v2
- run : echo ${{ github.sha }} # this outputs a SHA
- run : echo ${{ github.run_attempt }} # this outputs an int
- run: echo ${{ github.head_ref }} # outputs nothing
- run: echo ${{ github.base_ref }} # outputs nothing
- uses: actions/setup-python#v3
with:
python-version: '3.9.12'
name: Run black on diffed files
- run: echo ${{ github.head_ref }} # outputs nothing
- run: echo ${{ github.base_ref }} # outputs nothing
- run: pip install black && black $(git diff --name-only ${{ github.base_ref}} ${{ github.head_ref }} | grep .py)
The workflow successfully installs and runs black, but it fails because no files are being passed to the black command.
I'm not sure what I'm doing wrong here.

Answer 1

Found a solution that works for me, which also takes into consideration the advice of #jonrsharpe (see his comments on the original question).
Utilized another GitHub action to get all changed files. Then, I run black and reorder-python-imports only on the changed files. That way, the user knows which of their files have issues. However, for CI reasons (thanks Jon!), I also then run black on the full codebase using Black's official GHA.
NOTE: At present, the following GHA works as desired on pull-requests. However, if you push multiple commits, it only runs the action on the final commit in the group. I'm trying to figure out how to make it run on all commits that are pushed, rather than just the final one.
name: Black (python)
on: [push, pull_request]
jobs:
lint:
runs-on: ubuntu-latest
steps:
- name: Checkout branch
uses: actions/checkout#v3
with:
fetch-depth: 0
- name: Get changed files
id: changed-files
uses: tj-actions/changed-files#v35
with:
files: **/*.py
- name: Setup Python env
uses: actions/setup-python#v3
- name: Install black and reorder-python-imports
run: pip install black reorder-python-imports
- name: Black and Sort changed files
run: |
black --check reorder-python-imports ${{ steps.changed-files.outputs.all_changed_files }}
- name: Run black on full codebase
uses: psf/black#stable

Answer 2

This solution works for me at the moment - hope it matches what you need.
Basically, got from git the name of the changed files vis-a-vis master, and run black on them. Added some conditions and filters to only run it on non-deleted files and only python files:
changed_files=$(git diff --diff-filter=d --name-only $(git merge-base HEAD origin/master) HEAD | grep .py)
if [ ! -z "$changed_files" ];
then black -S --line-length 120 $(git diff --diff-filter=d --name-only $(git merge-base HEAD origin/master) HEAD | grep .py);
fi
And if you want to just check, add the relevant flag:
changed_files=$(git diff --diff-filter=d --name-only $(git merge-base HEAD origin/master) HEAD | grep .py)
if [ ! -z "$changed_files" ];
then black --check -S --line-length 120 $(git diff --diff-filter=d --name-only $(git merge-base HEAD origin/master) HEAD | grep .py);
fi