Robust Python

Part II. Defining Your Own Types

Welcome to Part II, where you’ll learn all about user-defined types. User-defined types are types that you, as a developer, create. In the first part of this book, I primarily focused on types that Python provides. However, these types are built for general use cases. They don’t tell you anything about the specific domain that you are operating in. In contrast, user-defined types serve as a conduit in which you express domain concepts in your codebase.

You need to build types that represent your domain. Python provides a few different ways to define your own data types, but you should take care with which one you choose. In this part of the book, we’ll go over three different user-defined types:

Enumerations (Enums)

Enumerations provide a developer with a restricted set of values.

Data classes

Data classes represent a relationship between different concepts.

Classes

Classes represent a relationship between different concepts, with an invariant that needs to be preserved.

You’ll learn all about using these types in a natural way and how they relate to one another. At the end of Part II, we’ll walk through modeling your domain data in a more natural way. The choices you make when designing your types are crucial. By learning the principles behind user-defined types, you will more effectively communicate to future developers.