This technique is known by several names: ports and adapters, hexagonal architecture, layered architecture, onion model, or (most boringly) dependency injection. The main idea is that you separate your business logic from your storage and from your presentation etc. so that you can easily swap out any single piece without refactoring all of your code. I originally read about this on Robert Martin’s site.

Here, I present a simple notes app using dependency injection for the storage and output. Right now I’m using TinyDB for storage, and presenting output to the terminal as a formatted string, or as JSON.

The first two abstract classes DB_Adapter and Output_Adapter define the general form what a database or output mechanism should have or provide. Next, we subclass these adapters into concrete classes that can pull data from an actual database, or present output in different ways. At the end of it all, when we instantiate the Notebook class, we pass if the database and output adapters that it will use in order to do its job. At this point, all of its dependencies have been provided (or injected) and it is free to focus on business logic, like managing permissions, spam filtering, or whatever.

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