How can I represent an ‘Enum’ in Python?

Enums have been added to Python 3.4 as described in PEP 435. It has also been backported to 3.3, 3.2, 3.1, 2.7, 2.6, 2.5, and 2.4 on pypi.

For more advanced Enum techniques try the aenum library (2.7, 3.3+, same author as enum34. Code is not perfectly compatible between py2 and py3, e.g. you’ll need __order__ in python 2).

• To use enum34, do $ pip install enum34
• To use aenum, do $ pip install aenum

Installing enum (no numbers) will install a completely different and incompatible version.

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from enum import Enum     # for enum34, or the stdlib version
# from aenum import Enum  # for the aenum version
Animal = Enum('Animal', 'ant bee cat dog')

Animal.ant  # returns <Animal.ant: 1>
Animal['ant']  # returns <Animal.ant: 1> (string lookup)
Animal.ant.name  # returns 'ant' (inverse lookup)

or equivalently:

class Animal(Enum):
    ant = 1
    bee = 2
    cat = 3
    dog = 4

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In earlier versions, one way of accomplishing enums is:

def enum(**enums):
    return type('Enum', (), enums)

which is used like so:

>>> Numbers = enum(ONE=1, TWO=2, THREE='three')
>>> Numbers.ONE
1
>>> Numbers.TWO
2
>>> Numbers.THREE
'three'

You can also easily support automatic enumeration with something like this:

def enum(*sequential, **named):
    enums = dict(zip(sequential, range(len(sequential))), **named)
    return type('Enum', (), enums)

and used like so:

>>> Numbers = enum('ZERO', 'ONE', 'TWO')
>>> Numbers.ZERO
0
>>> Numbers.ONE
1

Support for converting the values back to names can be added this way:

def enum(*sequential, **named):
    enums = dict(zip(sequential, range(len(sequential))), **named)
    reverse = dict((value, key) for key, value in enums.iteritems())
    enums['reverse_mapping'] = reverse
    return type('Enum', (), enums)

This overwrites anything with that name, but it is useful for rendering your enums in output. It will throw a KeyError if the reverse mapping doesn’t exist. With the first example:

>>> Numbers.reverse_mapping['three']
'THREE'

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If you are using MyPy another way to express “enums” is with typing.Literal.

For example:

from typing import Literal #python >=3.8
from typing_extensions import Literal #python 2.7, 3.4-3.7


Animal = Literal['ant', 'bee', 'cat', 'dog']

def hello_animal(animal: Animal):
    print(f"hello {animal}")

hello_animal('rock') # error
hello_animal('bee') # passes