Robust Python

Chapter 8. User-Defined Types: Enums

In this chapter, I’m going to focus on what a user-defined type is and cover the simplest user-defined data type: enumerations. I’ll discuss how to create an enumeration that will protect your code from common programming mistakes. I’ll then go over advanced features that allow you to express your ideas more clearly, such as creating aliases, making enumerations unique, or providing automatically generated values.

User-Defined Types

A user-defined type is a type that you, as a developer, create. You define what data is associated with the type and what behaviors are associated with your type. Each of these types should tie to a singular concept. This will help other developers build mental models about your codebase.

For instance, if I am writing restaurant point-of-sale systems, I would expect to come across concepts about the restaurant domain in your codebase. Concepts like restaurants, menu items, and tax calculations should all be naturally represented in code. If I were to use lists, dictionaries, and tuples instead, I’d force my readers to constantly reinterpret the meaning of variables to their more natural mappings.

Consider a simple function that calculates a total with tax. Which function would you rather work with?

def calculate_total_with_tax(restaurant: tuple[str, str, str, int],
                             subtotal: float) -> float:
    return subtotal * (1 + tax_lookup[restaurant[2]])

or

def calculate_total_with_tax(restaurant: Restaurant,
                             subtotal: decimal ...