Module Contents

This module defines two enumeration classes that can be used to define unique sets of names and values: Enum and IntEnum. It also defines one decorator, unique.

Enum

Base class for creating enumerated constants. See section Functional API for an alternate construction syntax.

IntEnum

Base class for creating enumerated constants that are also subclasses of int.

unique

Enum class decorator that ensures only one name is bound to any one value.

Creating an Enum

Enumerations are created using the class syntax, which makes them easy to read and write. An alternative creation method is described in Functional API. To define an enumeration, subclass Enum as follows:

>>> from enum import Enum
>>> class Color(Enum):
...     red = 1
...     green = 2
...     blue = 3

Note: Nomenclature

• The class Color is an enumeration (or enum)
• The attributes Color.red, Color.green, etc., are enumeration members (or enum members).
• The enum members have names and values (the name of Color.red is red, the value of Color.blue is 3, etc.)

Note:

Even though we use the class syntax to create Enums, Enums are not normal Python classes. See How are Enums different? for more details.

Enumeration members have human readable string representations:

>>> print(Color.red)
Color.red

...while their repr has more information:

>>> print(repr(Color.red))
<Color.red: 1>

The type of an enumeration member is the enumeration it belongs to:

>>> type(Color.red)
<enum 'Color'>
>>> isinstance(Color.green, Color)
True
>>>

Enum members also have a property that contains just their item name:

>>> print(Color.red.name)
red

Enumerations support iteration. In Python 3.x definition order is used; in Python 2.x the definition order is not available, but class attribute __order__ is supported; otherwise, value order is used:

>>> class Shake(Enum):
...   __order__ = 'vanilla chocolate cookies mint'  # only needed in 2.x
...   vanilla = 7
...   chocolate = 4
...   cookies = 9
...   mint = 3
...
>>> for shake in Shake:
...   print(shake)
...
Shake.vanilla
Shake.chocolate
Shake.cookies
Shake.mint

The __order__ attribute is always removed, and in 3.x it is also ignored (order is definition order); however, in the stdlib version it will be ignored but not removed.

Enumeration members are hashable, so they can be used in dictionaries and sets:

>>> apples = {}
>>> apples[Color.red] = 'red delicious'
>>> apples[Color.green] = 'granny smith'
>>> apples == {Color.red: 'red delicious', Color.green: 'granny smith'}
True

Programmatic access to enumeration members and their attributes

Sometimes it's useful to access members in enumerations programmatically (i.e. situations where Color.red won't do because the exact color is not known at program-writing time). Enum allows such access:

>>> Color(1)
<Color.red: 1>
>>> Color(3)
<Color.blue: 3>

If you want to access enum members by name, use item access:

>>> Color['red']
<Color.red: 1>
>>> Color['green']
<Color.green: 2>

If have an enum member and need its name or value:

>>> member = Color.red
>>> member.name
'red'
>>> member.value
1

Duplicating enum members and values

Having two enum members (or any other attribute) with the same name is invalid; in Python 3.x this would raise an error, but in Python 2.x the second member simply overwrites the first:

>>> # python 2.x
>>> class Shape(Enum):
...   square = 2
...   square = 3
...
>>> Shape.square
<Shape.square: 3>

>>> # python 3.x
>>> class Shape(Enum):
...   square = 2
...   square = 3
Traceback (most recent call last):
...
TypeError: Attempted to reuse key: 'square'

However, two enum members are allowed to have the same value. Given two members A and B with the same value (and A defined first), B is an alias to A. By-value lookup of the value of A and B will return A. By-name lookup of B will also return A:

>>> class Shape(Enum):
...   __order__ = 'square diamond circle alias_for_square'  # only needed in 2.x
...   square = 2
...   diamond = 1
...   circle = 3
...   alias_for_square = 2
...
>>> Shape.square
<Shape.square: 2>
>>> Shape.alias_for_square
<Shape.square: 2>
>>> Shape(2)
<Shape.square: 2>

Allowing aliases is not always desirable. unique can be used to ensure that none exist in a particular enumeration:

>>> from enum import unique
>>> @unique
... class Mistake(Enum):
...   __order__ = 'one two three four'  # only needed in 2.x
...   one = 1
...   two = 2
...   three = 3
...   four = 3
Traceback (most recent call last):
...
ValueError: duplicate names found in <enum 'Mistake'>: four -> three

Iterating over the members of an enum does not provide the aliases:

>>> list(Shape)
[<Shape.square: 2>, <Shape.diamond: 1>, <Shape.circle: 3>]

The special attribute __members__ is a dictionary mapping names to members. It includes all names defined in the enumeration, including the aliases:

>>> for name, member in sorted(Shape.__members__.items()):
...   name, member
...
('alias_for_square', <Shape.square: 2>)
('circle', <Shape.circle: 3>)
('diamond', <Shape.diamond: 1>)
('square', <Shape.square: 2>)

The __members__ attribute can be used for detailed programmatic access to the enumeration members. For example, finding all the aliases:

>>> [name for name, member in Shape.__members__.items() if member.name != name]
['alias_for_square']

Comparisons

Enumeration members are compared by identity:

>>> Color.red is Color.red
True
>>> Color.red is Color.blue
False
>>> Color.red is not Color.blue
True

Ordered comparisons between enumeration values are not supported. Enum members are not integers (but see IntEnum below):

>>> Color.red < Color.blue
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unorderable types: Color() < Color()

Equality comparisons are defined though:

>>> Color.blue == Color.red
False
>>> Color.blue != Color.red
True
>>> Color.blue == Color.blue
True

Comparisons against non-enumeration values will always compare not equal (again, IntEnum was explicitly designed to behave differently, see below):

>>> Color.blue == 2
False

Allowed members and attributes of enumerations

The examples above use integers for enumeration values. Using integers is short and handy (and provided by default by the Functional API), but not strictly enforced. In the vast majority of use-cases, one doesn't care what the actual value of an enumeration is. But if the value is important, enumerations can have arbitrary values.

Enumerations are Python classes, and can have methods and special methods as usual. If we have this enumeration:

>>> class Mood(Enum):
...   funky = 1
...   happy = 3
...
...   def describe(self):
...     # self is the member here
...     return self.name, self.value
...
...   def __str__(self):
...     return 'my custom str! {0}'.format(self.value)
...
...   @classmethod
...   def favorite_mood(cls):
...     # cls here is the enumeration
...     return cls.happy

Then:

>>> Mood.favorite_mood()
<Mood.happy: 3>
>>> Mood.happy.describe()
('happy', 3)
>>> str(Mood.funky)
'my custom str! 1'

The rules for what is allowed are as follows: _sunder_ names (starting and ending with a single underscore) are reserved by enum and cannot be used; all other attributes defined within an enumeration will become members of this enumeration, with the exception of __dunder__ names and descriptors (methods are also descriptors).

Note:

If your enumeration defines __new__ and/or __init__ then whatever value(s) were given to the enum member will be passed into those methods. See Planet for an example.

Restricted subclassing of enumerations

Subclassing an enumeration is allowed only if the enumeration does not define any members. So this is forbidden:

>>> class MoreColor(Color):
...   pink = 17
Traceback (most recent call last):
...
TypeError: Cannot extend enumerations

But this is allowed:

>>> class Foo(Enum):
...   def some_behavior(self):
...     pass
...
>>> class Bar(Foo):
...   happy = 1
...   sad = 2
...

Allowing subclassing of enums that define members would lead to a violation of some important invariants of types and instances. On the other hand, it makes sense to allow sharing some common behavior between a group of enumerations. (See OrderedEnum for an example.)

Pickling

Enumerations can be pickled and unpickled:

>>> from enum.test_enum import Fruit
>>> from pickle import dumps, loads
>>> Fruit.tomato is loads(dumps(Fruit.tomato, 2))
True

The usual restrictions for pickling apply: picklable enums must be defined in the top level of a module, since unpickling requires them to be importable from that module.

Note:

With pickle protocol version 4 (introduced in Python 3.4) it is possible to easily pickle enums nested in other classes.

Functional API

The Enum class is callable, providing the following functional API:

>>> Animal = Enum('Animal', 'ant bee cat dog')
>>> Animal
<enum 'Animal'>
>>> Animal.ant
<Animal.ant: 1>
>>> Animal.ant.value
1
>>> list(Animal)
[<Animal.ant: 1>, <Animal.bee: 2>, <Animal.cat: 3>, <Animal.dog: 4>]

The semantics of this API resemble namedtuple. The first argument of the call to Enum is the name of the enumeration.

The second argument is the source of enumeration member names. It can be a whitespace-separated string of names, a sequence of names, a sequence of 2-tuples with key/value pairs, or a mapping (e.g. dictionary) of names to values. The last two options enable assigning arbitrary values to enumerations; the others auto-assign increasing integers starting with 1. A new class derived from Enum is returned. In other words, the above assignment to Animal is equivalent to:

>>> class Animals(Enum):
...   ant = 1
...   bee = 2
...   cat = 3
...   dog = 4

Pickling enums created with the functional API can be tricky as frame stack implementation details are used to try and figure out which module the enumeration is being created in (e.g. it will fail if you use a utility function in separate module, and also may not work on IronPython or Jython). The solution is to specify the module name explicitly as follows:

>>> Animals = Enum('Animals', 'ant bee cat dog', module=__name__)

Derived Enumerations

>>> from enum import IntEnum
>>> class Shape(IntEnum):
...   circle = 1
...   square = 2
...
>>> class Request(IntEnum):
...   post = 1
...   get = 2
...
>>> Shape == 1
False
>>> Shape.circle == 1
True
>>> Shape.circle == Request.post
True

>>> class Shape(IntEnum):
...   circle = 1
...   square = 2
...
>>> class Color(Enum):
...   red = 1
...   green = 2
...
>>> Shape.circle == Color.red
False

>>> int(Shape.circle)
1
>>> ['a', 'b', 'c'][Shape.circle]
'b'
>>> [i for i in range(Shape.square)]
[0, 1]

class IntEnum(int, Enum):
    pass

Decorators

>>> @unique
... class NoDupes(Enum):
...    first = 'one'
...    second = 'two'
...    third = 'two'
Traceback (most recent call last):
...
ValueError: duplicate names found in <enum 'NoDupes'>: third -> second

Interesting examples

While Enum and IntEnum are expected to cover the majority of use-cases, they cannot cover them all. Here are recipes for some different types of enumerations that can be used directly, or as examples for creating one's own.

>>> class AutoNumber(Enum):
...     def __new__(cls):
...         value = len(cls.__members__) + 1
...         obj = object.__new__(cls)
...         obj._value_ = value
...         return obj
...
>>> class Color(AutoNumber):
...     __order__ = "red green blue"  # only needed in 2.x
...     red = ()
...     green = ()
...     blue = ()
...
>>> Color.green.value == 2
True

>>> class UniqueEnum(Enum):
...     def __init__(self, *args):
...         cls = self.__class__
...         if any(self.value == e.value for e in cls):
...             a = self.name
...             e = cls(self.value).name
...             raise ValueError(
...                     "aliases not allowed in UniqueEnum:  %r --> %r"
...                     % (a, e))
...
>>> class Color(UniqueEnum):
...     red = 1
...     green = 2
...     blue = 3
...     grene = 2
Traceback (most recent call last):
...
ValueError: aliases not allowed in UniqueEnum:  'grene' --> 'green'

>>> class OrderedEnum(Enum):
...     def __ge__(self, other):
...         if self.__class__ is other.__class__:
...             return self._value_ >= other._value_
...         return NotImplemented
...     def __gt__(self, other):
...         if self.__class__ is other.__class__:
...             return self._value_ > other._value_
...         return NotImplemented
...     def __le__(self, other):
...         if self.__class__ is other.__class__:
...             return self._value_ <= other._value_
...         return NotImplemented
...     def __lt__(self, other):
...         if self.__class__ is other.__class__:
...             return self._value_ < other._value_
...         return NotImplemented
...
>>> class Grade(OrderedEnum):
...     __ordered__ = 'A B C D F'
...     A = 5
...     B = 4
...     C = 3
...     D = 2
...     F = 1
...
>>> Grade.C < Grade.A
True

>>> class Planet(Enum):
...     MERCURY = (3.303e+23, 2.4397e6)
...     VENUS   = (4.869e+24, 6.0518e6)
...     EARTH   = (5.976e+24, 6.37814e6)
...     MARS    = (6.421e+23, 3.3972e6)
...     JUPITER = (1.9e+27,   7.1492e7)
...     SATURN  = (5.688e+26, 6.0268e7)
...     URANUS  = (8.686e+25, 2.5559e7)
...     NEPTUNE = (1.024e+26, 2.4746e7)
...     def __init__(self, mass, radius):
...         self.mass = mass       # in kilograms
...         self.radius = radius   # in meters
...     @property
...     def surface_gravity(self):
...         # universal gravitational constant  (m3 kg-1 s-2)
...         G = 6.67300E-11
...         return G * self.mass / (self.radius * self.radius)
...
>>> Planet.EARTH.value
(5.976e+24, 6378140.0)
>>> Planet.EARTH.surface_gravity
9.802652743337129

How are Enums different?

Enums have a custom metaclass that affects many aspects of both derived Enum classes and their instances (members).

>>> class FieldTypes(Enum):
...     name = 1
...     value = 2
...     size = 3
...
>>> FieldTypes.value.size
<FieldTypes.size: 3>
>>> FieldTypes.size.value
3

>>> dir(Planet)
['EARTH', 'JUPITER', 'MARS', 'MERCURY', 'NEPTUNE', 'SATURN', 'URANUS',
'VENUS', '__class__', '__doc__', '__members__', '__module__']
>>> dir(Planet.EARTH)
['__class__', '__doc__', '__module__', 'name', 'surface_gravity', 'value']