Gauging changes

A lot of the time, your change will affect formatting and/or performance. Quantifying these changes is hard, so we have tooling to help make it easier.

It’s recommended you evaluate the quantifiable changes your Black formatting modification causes before submitting a PR. Think about if the change seems disruptive enough to cause frustration to projects that are already “black formatted”.


diff-shades is a tool that runs Black across a list of open-source projects recording the results. The main highlight feature of diff-shades is being able to compare two revisions of Black. This is incredibly useful as it allows us to see what exact changes will occur, say merging a certain PR.

For more information, please see the diff-shades documentation.

CI integration

diff-shades is also the tool behind the “diff-shades results comparing …” / “diff-shades reports zero changes …” comments on PRs. The project has a GitHub Actions workflow that analyzes and compares two revisions of Black according to these rules:

Baseline revision

Target revision

On PRs

latest commit on main

PR commit with main merged

On pushes (main only)

latest PyPI version

the pushed commit

For pushes to main, there’s only one analysis job named preview-changes where the preview style is used for all projects.

For PRs they get one more analysis job: assert-no-changes. It’s similar to preview-changes but runs with the stable code style. It will fail if changes were made. This makes sure code won’t be reformatted again and again within the same year in accordance to Black’s stability policy.

Additionally for PRs, a PR comment will be posted embedding a summary of the preview changes and links to further information. If there’s a pre-existing diff-shades comment, it’ll be updated instead the next time the workflow is triggered on the same PR.


The preview-changes job will only fail intentionally if while analyzing a file failed to format. Otherwise a failure indicates a bug in the workflow.

The workflow uploads several artifacts upon completion:

  • The raw analyses (.json)

  • HTML diffs (.html)

  • .pr-comment.json (if triggered by a PR)

The last one is downloaded by the diff-shades-comment workflow and shouldn’t be downloaded locally. The HTML diffs come in handy for push-based where there’s no PR to post a comment. And the analyses exist just in case you want to do further analysis using the collected data locally.