I've only ever taken over a large Python project, never written it from scratch, and I don't have many years under my belt as a python coder. But my $.02: Docu

I've only ever taken over a large Python project, never written it from scratch, and I don't have many years under my belt as a python coder. But my $.02: Documentation, python code itself is easy to read but if you have a large project, broken into many small code bases for libraries, services and front ends then you need solid documentation. Not only text but also diagrams showing how all the parts work together. Not sure if they're called diagrams in english but it's the stuff you make in MS Visio or Draw.io.