Enum

In short: enum.Enum gives a value a fixed, named set of options instead of a loose string or number — Status.DONE instead of "done", spelled however anyone felt like that day. A typo in a string silently produces the wrong answer; a typo in an Enum member name raises AttributeError right where the mistake is. Members compare by identity (Light.RED is Light.RED), auto() assigns values so you don't have to pick numbers by hand, and IntEnum additionally behaves like a real int — comparable, sortable, usable wherever a number is expected. Reach for an Enum the moment a value can only ever be one of a short, known list.

A loose string "DONE" silently comparing False to a typo "DNOE" with no complaint, next to a traffic light housing three fixed named lamps RED/YELLOW/GREEN where Light.BLUE raises AttributeError because no fourth lamp exists

Order status without enum

A delivery app tracks each order's status as a string. It works — until one line checks the status against a misspelled string:

python

Nothing crashes. The archive step just silently never runs, and the bug could hide for months. An Enum turns the same typo into an error at the exact line that has it:

What happens when you run this?

from enum import Enum

class Status(Enum):
    PENDING = "pending"
    DONE = "done"
    FAILED = "failed"

print(Status.DNOE)

Two things worth knowing up front: an Enum member has both a .name and a .value, and it does not equal its raw value by default:

python

Traffic light states

A traffic light only ever has three states, and nobody wants to remember whether 1 means red or green. auto() hands out the numbers so you never have to pick them by hand:

python

auto() counts from 1 in definition order — reordering the lines changes the numbers, so never rely on the exact value if you used auto(), only on the name.

Priority levels that sort

A plain Enum gives you a fixed set, but not an order — two members can't be compared with < unless you ask for that specifically:

What happens when you run this?

from enum import Enum

class Priority(Enum):
    LOW = 1
    MEDIUM = 2
    HIGH = 3

print(Priority.LOW < Priority.HIGH)

IntEnum says so: a member is an int underneath, so ordering, sorting and even comparing to a plain number all just work:

python

Quick practice

What does this print?

from enum import Enum

class Suit(Enum):
    CLUBS = 1
    HEARTS = 2

print(Suit.CLUBS.value == 1, Suit(1) == Suit.CLUBS)

What does this print?

from enum import Enum

class Priority(Enum):
    LOW = 1
    MEDIUM = 2
    HIGH = 3

print([p.name for p in Priority])

broken — fix it

The function is supposed to work with either a string or a Status member. Right now only one of the two calls comes back True. Fix the comparison so both do.

🎯 Your turn

Write next_light(name). name is "RED", "GREEN" or "YELLOW" — the light's current state. Return the name of the next state in the cycle RED → GREEN → YELLOW → RED.

next_light("RED") → "GREEN"
next_light("YELLOW") → "RED"

Hint: define class Light(Enum): RED = 1; GREEN = 2; YELLOW = 3, keep ORDER = [Light.RED, Light.GREEN, Light.YELLOW], look up Light[name], find its index with .index(...), and step forward with % len(ORDER).

Press ▶ Run and hit ✓ Check. ✅

Enum

Enum

Order status without enum

Traffic light states

Priority levels that sort

Quick practice

🎯 Your turn

Practice — level up