Enums in Python

Some values come from a small fixed set: order status (pending, paid, shipped), user role (student, instructor, admin), day of week. Using raw strings like "shiped" invites typos that no tool catches. An enumeration defines the allowed values once as named constants.

This lesson covers Enum, auto(), StrEnum and IntEnum, Flag for combinable permissions, adding methods to enums, and using enums with match.

Defining and Using Enums

Subclass enum.Enum and list members as class attributes. Each member has a name and a value. Access members as Status.PAID, by value Status("paid") or by name Status["PAID"]. Members are singletons: compare with is or ==. Iterating the class yields members in definition order.

auto(), StrEnum and IntEnum

auto() assigns values automatically. StrEnum (3.11+) members are also strings — they serialise naturally to JSON and compare equal to their string values; with auto() the value is the lowercase member name. IntEnum members are integers. Plain Enum members are deliberately not equal to raw values, which catches mix-ups.

Flag for Combinations

enum.Flag members can be combined with | and tested with in — ideal for permissions: Permission.READ | Permission.WRITE.

Methods and Pattern Matching

Enums are classes, so they can have methods and properties, for example Status.can_cancel(). They work well with match statements to handle each state explicitly.

Examples

Defining, accessing and iterating an Enum

Python

from enum import Enum

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

s = Status.PAID
print(s, s.name, s.value)
print(Status("shipped"), Status["PENDING"])
print([m.name for m in Status])
print(s is Status.PAID, s == "paid")
try:
    Status("shiped")
except ValueError as e:
    print("ValueError:", e)

Output

Status.PAID PAID paid
Status.SHIPPED Status.PENDING
['PENDING', 'PAID', 'SHIPPED', 'CANCELLED']
True False
ValueError: 'shiped' is not a valid Status

auto(), StrEnum, IntEnum and JSON

Python

import json
from enum import Enum, IntEnum, StrEnum, auto

class Color(Enum):
    RED = auto()
    GREEN = auto()

class Role(StrEnum):
    STUDENT = auto()
    ADMIN = auto()

class Priority(IntEnum):
    LOW = 1
    HIGH = 3

print(Color.RED.value, Color.GREEN.value)
print(Role.ADMIN, Role.ADMIN == "admin", json.dumps({"role": Role.STUDENT}))
print(Priority.HIGH > Priority.LOW, Priority.HIGH + 1, sorted([Priority.HIGH, Priority.LOW]))

Output

1 2
admin True {"role": "student"}
True 4 [<Priority.LOW: 1>, <Priority.HIGH: 3>]

Flag permissions, enum methods and match

Python

from enum import Enum, Flag, auto

class Permission(Flag):
    READ = auto()
    WRITE = auto()
    DELETE = auto()

editor = Permission.READ | Permission.WRITE
print(Permission.WRITE in editor, Permission.DELETE in editor)

class OrderStatus(Enum):
    PENDING = "pending"
    SHIPPED = "shipped"
    DELIVERED = "delivered"

    def can_cancel(self):
        return self is OrderStatus.PENDING

def message(status):
    match status:
        case OrderStatus.PENDING:
            return "We are preparing your order."
        case OrderStatus.SHIPPED:
            return "Your order is on the way."
        case OrderStatus.DELIVERED:
            return "Delivered. Enjoy!"

for st in OrderStatus:
    print(f"{st.value:<10} cancel={st.can_cancel()!s:<5} {message(st)}")

Output

True False
pending    cancel=True  We are preparing your order.
shipped    cancel=False Your order is on the way.
delivered  cancel=False Delivered. Enjoy!

Common Mistakes

Comparing plain Enum members with raw strings (Status.PAID == "paid" is False) — use StrEnum or .value.
Using magic strings and numbers instead of enums for fixed sets of values.
Defining two members with the same value, which silently creates an alias; use @enum.unique to prevent it.
Storing enum names in a database and later renaming members.

Key Points to Remember

Enums define named constants for fixed sets of values.
Access members by attribute, value or name; iterate the class.
auto() generates values; StrEnum and IntEnum behave like str/int.
Flag supports combinable options with | and in.
Enums can have methods and work naturally with match.

Practice the examples

Change an input, predict the result, then compare it with the output. Explain why the result changes.

Try Defining, accessing and iterating an Enum in Webnest Codelab Try auto(), StrEnum, IntEnum and JSON in Webnest Codelab Try Flag permissions, enum methods and match in Webnest Codelab