Enum in Python

Difficulty: Intermediate

Question

What are Python Enums? When and why should you use them over plain constants?

Answer

Enum (Python 3.4+) creates a set of symbolic, named constants grouped in a class. Members are instances of the Enum class, not bare integers or strings.

Advantages over constants (STATUS_ACTIVE = 1): - Type safety: passing the wrong value is harder - Auto-complete and IDE support - Iteration, membership testing, comparison - Descriptive __repr__ - Cannot accidentally reuse values (unless using aliases)

Variants: - IntEnum: members compare equal to integers - StrEnum (3.11+): members compare equal to strings - Flag/IntFlag: bitmask operations (|, &, ~) - auto(): auto-generates values

Code examples

Basic Enum Usage

from enum import Enum, auto, Flag

class Status(Enum):
    PENDING = auto()
    ACTIVE = auto()
    INACTIVE = auto()
    DELETED = auto()

# Access
print(Status.ACTIVE)          # Status.ACTIVE
print(Status.ACTIVE.name)     # ACTIVE
print(Status.ACTIVE.value)    # 2 (auto starts at 1)
print(repr(Status.ACTIVE))    # <Status.ACTIVE: 2>

# Lookup
print(Status['PENDING'])      # Status.PENDING (by name)
print(Status(2))              # Status.ACTIVE (by value)

# Comparison
print(Status.ACTIVE == Status.ACTIVE)    # True
print(Status.ACTIVE == Status.INACTIVE)  # False
print(Status.ACTIVE == 2)                # False! (not IntEnum)

# Iteration
for s in Status:
    print(f'{s.name} = {s.value}')

# Membership
print(Status.ACTIVE in Status)  # True

# Use in type hints
def update_user(user_id: int, status: Status) -> None:
    pass

update_user(1, Status.ACTIVE)  # Clear intent

auto() generates sequential values. Enum members do not compare equal to plain integers (use IntEnum for that).

IntEnum and Flag

from enum import IntEnum, Flag, auto

# IntEnum: works like int
class Priority(IntEnum):
    LOW = 1
    MEDIUM = 2
    HIGH = 3

print(Priority.HIGH > Priority.LOW)  # True
print(Priority.HIGH + 1)              # 4 (int arithmetic)

tasks = [
    {'name': 'Task A', 'priority': Priority.HIGH},
    {'name': 'Task B', 'priority': Priority.LOW},
]
print(sorted(tasks, key=lambda t: t['priority'], reverse=True))

# Flag: bitmask permissions
class Permission(Flag):
    READ = auto()    # 1
    WRITE = auto()   # 2
    EXECUTE = auto() # 4
    ADMIN = READ | WRITE | EXECUTE  # 7

user_perms = Permission.READ | Permission.WRITE
print(user_perms)                           # Permission.READ|WRITE
print(Permission.READ in user_perms)        # True
print(Permission.EXECUTE in user_perms)     # False

# Toggle permission
user_perms ^= Permission.WRITE  # Remove WRITE
print(Permission.WRITE in user_perms)       # False

IntEnum inherits from int, enabling arithmetic and comparison with integers. Flag enables bitwise combination of permission flags.

Key points

• StrEnum (Python 3.11+): members compare equal to strings and lower() by default
• Enum aliases: two names with same value are aliases, not separate members
• @unique decorator prevents accidental aliases
• Enum members are singletons - identity comparison (is) works: Status.ACTIVE is Status.ACTIVE

Concepts covered

Enum, IntEnum, auto(), Flag, Value/Name