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I was curious to see how much this saved, so I looked at the recent history on pre-commit.ci
The run for this PR isn't hard to spot !

We do have that file from when we initially implemented https://github.com/astropy/astropy-APEs/blob/main/APE20.rst but since then, we have not been adding follow-up ruff/black updates to it. Regardless, I don't think we can do this until we get the final merge commit after merging this PR? Anyways, if you want something added to https://github.com/astropy/astropy/blob/main/.git-blame-ignore-revs , either we do follow-up PR or I can do it directly on main as needed. Thanks!

I don't think we can do this until we get the final merge commit after merging this PR?

Only if we squash merge, but as you said, it can be done in a follow up !

2. We have configured Black to ignore docs/ and examples/. I think it would be a good idea to start formatting Python code in those directories too, but not in this pull request.

I will open a new PR for that. Copied over the black exclude list to ruff format too.

3. If we are not using Black anymore then all the Black configuration in pyproject.toml should be converted to equivalent Ruff formatter configuration.

Done.

4. Our developer documentation needs to be updated.

I have updated it in the places I saw black pop up. I have still put in links to the OG black style guide as ruff format wants to use that as the base and not deviate too much from it and the black docs explain it much better than ruff's docs.

Not so fast, 3 warnings in doc build:

docs/development/codeguide.rst:575: WARNING: Duplicate explicit target name: "ruff".
docs/development/codeguide.rst:168: ERROR: Duplicate target name, cannot be used as a unique reference: "ruff".
docs/development/codeguide.rst:172: ERROR: Duplicate target name, cannot be used as a unique reference: "ruff".

7 approvals! This must be a record. 😸