I am a big fan of Build Your Own X educational projects. Recently, I released a Go version of my build your own KV Store project. I have set up this project in TDD fashion with the tests. So, you start with simple functions, pass the tests, and the difficulty level goes up. There are hints if you get stuck (e.g. link). When all the tests pass, you will have written a persistent key-value store.

go - https://github.com/avinassh/go-caskdb

python - https://github.com/avinassh/py-caskdb