PEP 462 is an interesting insight into the challenges with Python core development. We're spoilt with GitHub, Travis and pull requests.
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I'm increasingly doubtful that commit messages should be entirely immutable.
I sometimes find myself editing or commenting on merged pull requests, to help future readers.
Phabricator appended a URL to commit messages, which helped. In principle PR merging could do the same thing.
Learning software development through playing and contributing to a MUD: https://tashian.com/articles/how-i-learned-to-program/
"Example Driven Development" using Glamorous and Pharo Smalltalk: https://medium.com/feenk/an-example-of-example-driven-development-4dea0d995920
Tests returning values and composing is a really interesting model. It establishes structure and shows which test failure is the most 'fundamental'.