Simon Willison’s Weblog

Add tests in a commit before the fix. They should pass, showing the behavior before your change. Then, the commit with your change will update the tests. The diff between these commits represents the change in behavior. This helps the author test their tests (I've written tests thinking they covered the relevant case but didn't), the reviewer to more precisely see the change in behavior and comment on it, and the wider community to understand what the PR description is about.

— Ed Page