Enums

So far, we've used classes to model the different cases in a sum type, and union type hints as a simpler way of describing the possible types of a value (albeit with no automatic enforcement at runtime).

If what you're trying to represent is a fixed set of values, you have another good option in Python's type system: enums.

Let's say we have a Color variable that we want to restrict to only three possible values:

• RED
• GREEN
• BLUE

We could use a plain old str to represent these values, but that's annoying because we have to keep track of the "valid" values and defensively check for invalid ones all over our codebase. Instead, we can use an Enum:

from enum import Enum

Color = Enum("Color", ["RED", "GREEN", "BLUE"])
print(Color.RED)  # this works, prints 'Color.RED'
print(Color.TEAL)  # this raises an exception

There is also a manual class-based syntax:

from enum import Enum


class Color(Enum):
    RED = 1
    GREEN = 2
    BLUE = 3


print(Color.RED)  # this works, prints 'Color.RED'
print(Color.TEAL)  # this raises an exception

Now Color is a sum type! At least, as close as we can get in Python. There are a few benefits:

1. A Color can only be RED, GREEN, or BLUE. If you try to use Color.TEAL, Python raises an exception.
2. There is a central place to see the "valid" values for a Color.
3. Each Color has a "name" (e.g. RED) and an integer value (e.g. 1). The value can be useful if you need to store, compare, or serialize the enum in a specific way.