8.13. enum — Support for enumerations

Source code: Lib/enum.py

---

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.

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(), that ensures only unique member values are present in an enumeration.

8.13.1. 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 definition order:

>>> class Shake(Enum):
...     vanilla = 7
...     chocolate = 4
...     cookies = 9
...     mint = 3
...
>>> for shake in Shake:
...     print(shake)
...
Shake.vanilla
Shake.chocolate
Shake.cookies
Shake.mint

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

8.13.2. 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

8.13.3. Duplicating enum members and values

Having two enum members with the same name is invalid:

>>> 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):
...     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>

Note

Attempting to create a member with the same name as an already defined attribute (another member, a method, etc.) or attempting to create an attribute with the same name as a member is not allowed.

8.13.4. Ensuring unique enumeration values

By default, enumerations allow multiple names as aliases for the same value. When this behavior isn’t desired, the following decorator can be used to ensure each value is used only once in the enumeration:

@enum.unique

A class decorator specifically for enumerations. It searches an enumeration’s __members__ gathering any aliases it finds; if any are found ValueError is raised with the details:

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

8.13.5. Iteration

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 an ordered dictionary mapping names to members. It includes all names defined in the enumeration, including the aliases:

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

8.13.6. 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

8.13.7. 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.

8.13.8. 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.)

8.13.9. Pickling

Enumerations can be pickled and unpickled:

>>> from test.test_enum import Fruit
>>> from pickle import dumps, loads
>>> Fruit.tomato is loads(dumps(Fruit.tomato))
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.

Warning

In order to support the singleton nature of enumeration members, pickle protocol version 2 or higher must be used.

8.13.10. 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
...

The reason for defaulting to 1 as the starting number and not 0 is that 0 is False in a boolean sense, but enum members all evaluate to True.

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__)

8.13.11. Derived Enumerations

8.13.11.1. IntEnum

A variation of Enum is provided which is also a subclass of int. Members of an IntEnum can be compared to integers; by extension, integer enumerations of different types can also be compared to each other:

>>> 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

However, they still can’t be compared to standard Enum enumerations:

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

IntEnum values behave like integers in other ways you’d expect:

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

For the vast majority of code, Enum is strongly recommended, since IntEnum breaks some semantic promises of an enumeration (by being comparable to integers, and thus by transitivity to other unrelated enumerations). It should be used only in special cases where there’s no other choice; for example, when integer constants are replaced with enumerations and backwards compatibility is required with code that still expects integers.

8.13.11.2. Others

While IntEnum is part of the enum module, it would be very simple to implement independently:

class IntEnum(int, Enum):
    pass

This demonstrates how similar derived enumerations can be defined; for example a StrEnum that mixes in str instead of int.

Some rules:

1. When subclassing Enum, mix-in types must appear before Enum itself in the sequence of bases, as in the IntEnum example above.
2. While Enum can have members of any type, once you mix in an additional type, all the members must have values of that type, e.g. int above. This restriction does not apply to mix-ins which only add methods and don’t specify another data type such as int or str.
3. When another data type is mixed in, the value attribute is not the same as the enum member itself, although it is equivalant and will compare equal.
4. %-style formatting: %s and %r call Enum‘s __str__() and __repr__() respectively; other codes (such as %i or %h for IntEnum) treat the enum member as its mixed-in type.
5. str.__format__() (or format()) will use the mixed-in type’s __format__(). If the Enum‘s str() or repr() is desired use the !s or !r str format codes.

8.13.12. 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.

8.13.12.1. AutoNumber

Avoids having to specify the value for each enumeration member:

>>> class AutoNumber(Enum):
...     def __new__(cls):
...         value = len(cls.__members__) + 1
...         obj = object.__new__(cls)
...         obj._value_ = value
...         return obj
...
>>> class Color(AutoNumber):
...     red = ()
...     green = ()
...     blue = ()
...
>>> Color.green.value == 2
True

Note

The __new__() method, if defined, is used during creation of the Enum members; it is then replaced by Enum’s __new__() which is used after class creation for lookup of existing members. Due to the way Enums are supposed to behave, there is no way to customize Enum’s __new__().

8.13.12.2. OrderedEnum

An ordered enumeration that is not based on IntEnum and so maintains the normal Enum invariants (such as not being comparable to other enumerations):

>>> 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):
...     A = 5
...     B = 4
...     C = 3
...     D = 2
...     F = 1
...
>>> Grade.C < Grade.A
True

8.13.12.3. DuplicateFreeEnum

Raises an error if a duplicate member name is found instead of creating an alias:

>>> class DuplicateFreeEnum(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 DuplicateFreeEnum:  %r --> %r"
...                 % (a, e))
...
>>> class Color(DuplicateFreeEnum):
...     red = 1
...     green = 2
...     blue = 3
...     grene = 2
...
Traceback (most recent call last):
...
ValueError: aliases not allowed in DuplicateFreeEnum:  'grene' --> 'green'

Note

This is a useful example for subclassing Enum to add or change other behaviors as well as disallowing aliases. If the only desired change is disallowing aliases, the unique() decorator can be used instead.

8.13.12.4. Planet

If __new__() or __init__() is defined the value of the enum member will be passed to those methods:

>>> 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

8.13.13. How are Enums different?

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

8.13.13.1. Enum Classes

The EnumMeta metaclass is responsible for providing the __contains__(), __dir__(), __iter__() and other methods that allow one to do things with an Enum class that fail on a typical class, such as list(Color) or some_var in Color. EnumMeta is responsible for ensuring that various other methods on the final Enum class are correct (such as __new__(), __getnewargs__(), __str__() and __repr__())

8.13.13.2. Enum Members (aka instances)

The most interesting thing about Enum members is that they are singletons. EnumMeta creates them all while it is creating the Enum class itself, and then puts a custom __new__() in place to ensure that no new ones are ever instantiated by returning only the existing member instances.

8.13.13.3. Finer Points

Enum members are instances of an Enum class, and even though they are accessible as EnumClass.member, they are not accessible directly from the member:

>>> Color.red
<Color.red: 1>
>>> Color.red.blue
Traceback (most recent call last):
...
AttributeError: 'Color' object has no attribute 'blue'

Likewise, the __members__ is only available on the class.

If you give your Enum subclass extra methods, like the Planet class above, those methods will show up in a dir() of the member, but not of the class:

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

A __new__() method will only be used for the creation of the Enum members – after that it is replaced. This means if you wish to change how Enum members are looked up you either have to write a helper function or a classmethod().