At the end of Harry’s last book, Test-Driven Development with Python (O’Reilly), he found himself asking a bunch of questions about architecture, such as, What’s the best way of structuring your application so that it’s easy to test? More specifically, so that your core business logic is covered by unit tests, and so that you minimize the number of integration and end-to-end tests you need? He made vague references to «Hexagonal Architecture» and «Ports and Adapters» and «Functional Core, Imperative Shell,» but if he was honest, he’d have to admit that these weren’t things he really understood or had done in practice.