Working through Avdi Grime's Mastering the Object Oriented Mindset.
I liked the lessons in the videos, but wanted to take a spin and writing the code myself. I first wrote everything out in a giant multi-class file, and added tests along side, then did this - splitting the classes apart, testing correctly, etc.
More info on what this thing is here:
https://josh.works/primative-obsession-and-exceptional-values