Programming Fundamentals › Type Systems

Enum

A type with a fixed set of named values.

Also known as: enumeration, enumerated type, enums

An enum (enumeration) is a type with a fixed set of named values. It replaces loose strings and numbers with choices the program can check.

from enum import Enum

class Status(Enum):
    PENDING = "pending"
    PAID = "paid"
    SHIPPED = "shipped"

order.status = Status.PAID
if order.status == Status.SHIPPED:
    ...
Status("paid")             # Status.PAID, from stored text
Status.PAID.value          # "paid"

enum Status { PENDING, PAID, SHIPPED }

enum Status { Pending = "pending", Paid = "paid" }
type Status2 = "pending" | "paid";       // a string union is a common alternative

Why not plain strings or numbers

• No typos. Status.PAD is an error. "pad" is a quiet bug.
• Clear meaning. Status.PAID beats 2 (magic numbers).
• Autocomplete lists valid options.
• Exhaustiveness. The compiler or linter can warn if a switch forgets a value (switch).

Practical points

• Store a stable value, not a position. If you save 1 for “PAID” and later insert a value in the middle, old data changes meaning. Give each value an explicit, permanent code.
• Handle unknown values coming from outside (an API sends a status you don’t know), instead of crashing.
• Adding values is easy. Removing or renaming them can break stored data and other systems.
• Databases can store them as text with a check constraint, a lookup table, or a native enum type, each with trade-offs (DB enums).
• Don’t use an enum for values that change often (a list of countries or products). That’s data, not code.
• For values that carry different data per variant, see discriminated unions.