Add more tests on the pre-commit hook

@JohnPaton and @Skylion007 have developed a pre-commit hook for this repo - that was an impressive work #698!

Here is a list of small improvements that we identified during the PR and that I’d like to include in the next release:

• Add an entry to the CHANGELOG
• Update the documentation for the --to test to match the example that is actually tested
• In the documentation, the emphasis should be on the hook for the pre-commit framework. We could move the documentation for the manual pre-commit hook to another section.
• Split the tests on the pre-commit into multiple files. Again the emphasis should be on the new tests and on the use cases documented in the documentation
• Refactor the tests to make them easier to read, using fixtures. If possible, test the error text (currently we just test SystemExit which does not contain a detailed error message)
• Change the jupytext CLI to avoid writing files that don’t change (only update their timestamp). And make sure that this fixes the pre-commit hook when run on file types .py + .ipynb (or even tripled-paired notebooks)
• Test and document a --sync --pipe black mode.
• Test and document a --sync + nbstripout mode.
• Test and document a --sync --execute mode.
• Find a way to safely reformat long markdown cells (without breaking markdown tables…)
• Test modifying the .py file in the --sync mode
• Test the hook in the context of pre-commit run --all (no need to add files to the git index in that context)
• Test moving a paired notebook

[mwouts]

Wow! Congrats John!

Well as you see we finally were able to ship the pre-commit hook in Jupytext v1.10.0. I have made no announcement yet, in part because I wanted to finish the blog post, and also because I wanted to find the appropriate approach for the new cell ids in nbformat 5.1.1 (everything seems fine in the end if one does either --update or --sync, from Jupytext 1.7.1 on).

I hope to come back to you both soon with more updates on the article, but that might take me a few more days/weeks… Slow progresses, but this is a very nice feature, it is worth documenting it well, isn’t it?

[mwouts]

I had more fun with this tonight!

• I moved each use case to a new test file to improve the readability of tests (not sure however how many people actually read the tests 😄 )
• --sync and nbstripout work well together, see test_pre_commit_2_sync_nbstripout.py
• --sync and nbstripout and black also seem to interact well, however as @JohnPaton previously noticed black needs to be present in jupytext --pipe, cf test_pre_commit_3_sync_black_nbstripout.py
• maybe I’d like an example of a hook that does not commit the ipynb file, but I don’t see how it could work… is it possible for a hook to reset files in the index?
• I still have to handle a side effect in test 3 - the global pairing causes a modification on the py file the second time the hook runs, see the TODOs there
• --execute almost works. Thanks to the new --diff argument that displays the change when the file has to be rewritten, we clearly see that this is because of execution timestamps… Do you agree that in --execute --pre-commit-mode we can consider that the notebook has not changed if only the execution timestamps change?