Python 3: An Intro to Enumerations

Python added the enum module to the standard library in version 3.4. The Python documentation describes an enum like this:

An enumeration is a set of symbolic names (members) bound to unique, constant values. Within an enumeration, the members can be compared by identity, and the enumeration itself can be iterated over.

Let’s look at how you might create an Enum object:

>>> from enum import Enum
>>> class AnimalEnum(Enum):
        HORSE = 1
        COW = 2
        CHICKEN = 3
        DOG = 4
>>> print(AnimalEnum.CHICKEN)
AnimalEnum.CHICKEN
>>> print(repr(AnimalEnum.CHICKEN))
<AnimalEnum.CHICKEN: 3>

Here we create an Enumeration class called AnimalEnum. Inside the class, we create class attributes called enumeration members, which are constants. When you try to print out an enum member, you will just get back the same string. But if you print out the repr of the enum member, you will get the enum member and its value.

If you try to modify an enum member, Python will raise an AttributeError:

>>> AnimalEnum.CHICKEN = 5
Traceback (most recent call last):
  Python Shell, prompt 5, line 1
  File "C:\Users\mike\AppData\Local\Programs\PYTHON\PYTHON36-32\Lib\enum.py", line 361, in __setattr__
    raise AttributeError('Cannot reassign members.')
builtins.AttributeError: Cannot reassign members.

Enum members have some properties you can use to get their name and value:

>>> AnimalEnum.CHICKEN.name
'CHICKEN'
>>> AnimalEnum.CHICKEN.value
3

Enumerations also support iteration. So you can do something fun like this:

>>> for animal in AnimalEnum:
        print('Name: {}  Value: {}'.format(animal, animal.value))
 
Name: AnimalEnum.HORSE  Value: 1
Name: AnimalEnum.COW  Value: 2
Name: AnimalEnum.CHICKEN  Value: 3
Name: AnimalEnum.DOG  Value: 4

Python’s enumerations do not allow you to create enum members with the same name:

>>> class Shapes(Enum):
...     CIRCLE = 1
...     SQUARE = 2
...     SQUARE = 3
... 
Traceback (most recent call last):
  Python Shell, prompt 13, line 1
  Python Shell, prompt 13, line 4
  File "C:\Users\mike\AppData\Local\Programs\PYTHON\PYTHON36-32\Lib\enum.py", line 92, in __setitem__
    raise TypeError('Attempted to reuse key: %r' % key)
builtins.TypeError: Attempted to reuse key: 'SQUARE'

As you can see, when you try to reuse an enum member name, it will raise a TypeError.

You can also create an Enum like this:

>>> AnimalEnum = Enum('Animal', 'HORSE COW CHICKEN DOG')
>>> AnimalEnum
<enum 'Animal'>
>>> AnimalEnum.CHICKEN
<Animal.CHICKEN: 3>

Personally, I think that is really neat!


Enum Member Access

Interestingly enough, there are multiple ways to access enum members. For example, if you don’t know which enum is which, you can just call the enum directly and pass it a value:

>>> AnimalEnum(2)
<AnimalEnum.COW: 2>

If you happen to pass in an invalid value, then Python raises a ValueError

>>> AnimalEnum(8)
Traceback (most recent call last):
  Python Shell, prompt 11, line 1
  File "C:\Users\mike\AppData\Local\Programs\PYTHON\PYTHON36-32\Lib\enum.py", line 291, in __call__
    return cls.__new__(cls, value)
  File "C:\Users\mike\AppData\Local\Programs\PYTHON\PYTHON36-32\Lib\enum.py", line 533, in __new__
    return cls._missing_(value)
  File "C:\Users\mike\AppData\Local\Programs\PYTHON\PYTHON36-32\Lib\enum.py", line 546, in _missing_
    raise ValueError("%r is not a valid %s" % (value, cls.__name__))
builtins.ValueError: 8 is not a valid AnimalEnum

You may also access an enumeration by name:

>>> AnimalEnum['CHICKEN']
<AnimalEnum.CHICKEN: 3>

Enum Niceties

The enum module also has a couple of other fun things you can import. For example, you can can create automatic values for your enumerations:

>>> from enum import auto, Enum
>>> class Shapes(Enum):
        CIRCLE = auto()
        SQUARE = auto()
        OVAL = auto() 
>>> Shapes.CIRCLE
<Shapes.CIRCLE: 1>

You can also import a handy enum decorator to make sure that your enumeration members are unique:

>>> @unique
    class Shapes(Enum):
        CIRCLE = 1
        SQUARE = 2
        TRIANGLE = 1
 
Traceback (most recent call last):
  Python Shell, prompt 18, line 2
  File "C:\Users\mike\AppData\Local\Programs\PYTHON\PYTHON36-32\Lib\enum.py", line 830, in unique
    (enumeration, alias_details))
builtins.ValueError: duplicate values found in <enum 'Shapes'>: TRIANGLE -> CIRCLE

Here we create an enumeration that has two members trying to map to the same value. Because we added the @unique decorator, a ValueError is raised if there are any duplicate values in the enum members. If you do not have the @unique decorator applied, then you can have enum members with the same value.


Wrapping Up

While I don’t think the enum module is really necessary for Python, it is a neat tool to have available. The documentation has a lot more examples and demonstrates other types of enums, so it is definitely worth a read.


Further Reading

Print Friendly, PDF & Email