(self-promotion) I've created an open source C compiler written in Python (https://github.com/ShivamSarodia/ShivyC). I've learned a ton building it out so far, and I've found compiler design to be a great exercise in software engineering and learning Python since managing the complexity of a compiler is nontrivial. If you're interested in contributing, a good place to start is by tackling any of the issues marked as "good first issue".