`enum` — Support for enumerations¶

New in version 3.4.

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.

Note

Case of Enum Members

Because Enums are used to represent constants we recommend using UPPER_CASE names for enum members, and will be using that style in our examples.

Module Contents¶

This module defines four enumeration classes that can be used to define unique sets of names and values: Enum, IntEnum, Flag, and IntFlag. It also defines one decorator, unique(), and one helper, auto.

class enum.Enum¶: Base class for creating enumerated constants. See section Functional API for an alternate construction syntax.

class enum.IntEnum¶: Base class for creating enumerated constants that are also subclasses of int.

class enum.IntFlag¶: Base class for creating enumerated constants that can be combined using the bitwise operators without losing their IntFlag membership. IntFlag members are also subclasses of int.

class enum.Flag¶: Base class for creating enumerated constants that can be combined using the bitwise operations without losing their Flag membership.

enum.unique(): Enum class decorator that ensures only one name is bound to any one value.

class enum.auto¶: Instances are replaced with an appropriate value for Enum members. By default, the initial value starts at 1.

New in version 3.6: Flag, IntFlag, auto

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

Enum member values

Member values can be anything: int, str, etc.. If the exact value is unimportant you may use auto instances and an appropriate value will be chosen for you. Care must be taken if you mix auto with other values.

Note

Nomenclature

The class Color is an enumeration (or enum)
The attributes Color.RED, Color.GREEN, etc., are enumeration members (or enum members) and are functionally constants.
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

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 you 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 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.

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

Using automatic values¶

If the exact value is unimportant you can use auto:

>>> from enum import Enum, auto
>>> class Color(Enum):
...     RED = auto()
...     BLUE = auto()
...     GREEN = auto()
...
>>> list(Color)
[<Color.RED: 1>, <Color.BLUE: 2>, <Color.GREEN: 3>]

The values are chosen by _generate_next_value_(), which can be overridden:

>>> class AutoName(Enum):
...     def _generate_next_value_(name, start, count, last_values):
...         return name
...
>>> class Ordinal(AutoName):
...     NORTH = auto()
...     SOUTH = auto()
...     EAST = auto()
...     WEST = auto()
...
>>> list(Ordinal)
[<Ordinal.NORTH: 'NORTH'>, <Ordinal.SOUTH: 'SOUTH'>, <Ordinal.EAST: 'EAST'>, <Ordinal.WEST: 'WEST'>]

Note

The goal of the default _generate_next_value_() method is to provide the next int in sequence with the last int provided, but the way it does this is an implementation detail and may change.

Note

The _generate_next_value_() method must be defined before any members.

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 a read-only ordered mapping of 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']

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: '<' not supported between instances of 'Color' and '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: names that start and end 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 special methods (__str__(), __add__(), etc.), descriptors (methods are also descriptors), and variable names listed in _ignore_.

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

Restricted Enum subclassing¶

A new Enum class must have one base Enum class, up to one concrete data type, and as many object-based mixin classes as needed. The order of these base classes is:

class EnumName([mix-in, ...,] [data-type,] base-enum):
    pass

Also, 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: MoreColor: cannot extend enumeration 'Color'

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 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.

Note

With pickle protocol version 4 it is possible to easily pickle enums nested in other classes.

It is possible to modify how Enum members are pickled/unpickled by defining __reduce_ex__() in the enumeration class.

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 (use the start parameter to specify a different starting value). A new class derived from Enum is returned. In other words, the above assignment to Animal is equivalent to:

>>> class Animal(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:

>>> Animal = Enum('Animal', 'ANT BEE CAT DOG', module=__name__)

Warning

If module is not supplied, and Enum cannot determine what it is, the new Enum members will not be unpicklable; to keep errors closer to the source, pickling will be disabled.

The new pickle protocol 4 also, in some circumstances, relies on __qualname__ being set to the location where pickle will be able to find the class. For example, if the class was made available in class SomeData in the global scope:

>>> Animal = Enum('Animal', 'ANT BEE CAT DOG', qualname='SomeData.Animal')

The complete signature is:

Enum(value='NewEnumName', names=<...>, *, module='...', qualname='...', type=<mixed-in class>, start=1)

value

What the new Enum class will record as its name.

names

The Enum members. This can be a whitespace or comma separated string (values will start at 1 unless otherwise specified):

'RED GREEN BLUE' | 'RED,GREEN,BLUE' | 'RED, GREEN, BLUE'

or an iterator of names:

['RED', 'GREEN', 'BLUE']

or an iterator of (name, value) pairs:

[('CYAN', 4), ('MAGENTA', 5), ('YELLOW', 6)]

or a mapping:

{'CHARTREUSE': 7, 'SEA_GREEN': 11, 'ROSEMARY': 42}

module

name of module where new Enum class can be found.

qualname

where in module new Enum class can be found.

type

type to mix in to new Enum class.

start

number to start counting at if only names are passed in.

Changed in version 3.5: The start parameter was added.

Derived Enumerations¶

IntEnum¶

The first variation of Enum that is provided 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]

IntFlag¶

The next variation of Enum provided, IntFlag, is also based on int. The difference being IntFlag members can be combined using the bitwise operators (&, |, ^, ~) and the result is still an IntFlag member. However, as the name implies, IntFlag members also subclass int and can be used wherever an int is used. Any operation on an IntFlag member besides the bit-wise operations will lose the IntFlag membership.

New in version 3.6.

Sample IntFlag class:

>>> from enum import IntFlag
>>> class Perm(IntFlag):
...     R = 4
...     W = 2
...     X = 1
...
>>> Perm.R | Perm.W
<Perm.R|W: 6>
>>> Perm.R + Perm.W
6
>>> RW = Perm.R | Perm.W
>>> Perm.R in RW
True

It is also possible to name the combinations:

>>> class Perm(IntFlag):
...     R = 4
...     W = 2
...     X = 1
...     RWX = 7
>>> Perm.RWX
<Perm.RWX: 7>
>>> ~Perm.RWX
<Perm.-8: -8>

Another important difference between IntFlag and Enum is that if no flags are set (the value is 0), its boolean evaluation is False:

>>> Perm.R & Perm.X
<Perm.0: 0>
>>> bool(Perm.R & Perm.X)
False

Because IntFlag members are also subclasses of int they can be combined with them:

>>> Perm.X | 8
<Perm.8|X: 9>

Flag¶

The last variation is Flag. Like IntFlag, Flag members can be combined using the bitwise operators (&, |, ^, ~). Unlike IntFlag, they cannot be combined with, nor compared against, any other Flag enumeration, nor int. While it is possible to specify the values directly it is recommended to use auto as the value and let Flag select an appropriate value.

New in version 3.6.

Like IntFlag, if a combination of Flag members results in no flags being set, the boolean evaluation is False:

>>> from enum import Flag, auto
>>> class Color(Flag):
...     RED = auto()
...     BLUE = auto()
...     GREEN = auto()
...
>>> Color.RED & Color.GREEN
<Color.0: 0>
>>> bool(Color.RED & Color.GREEN)
False

Individual flags should have values that are powers of two (1, 2, 4, 8, …), while combinations of flags won’t:

>>> class Color(Flag):
...     RED = auto()
...     BLUE = auto()
...     GREEN = auto()
...     WHITE = RED | BLUE | GREEN
...
>>> Color.WHITE
<Color.WHITE: 7>

Giving a name to the “no flags set” condition does not change its boolean value:

>>> class Color(Flag):
...     BLACK = 0
...     RED = auto()
...     BLUE = auto()
...     GREEN = auto()
...
>>> Color.BLACK
<Color.BLACK: 0>
>>> bool(Color.BLACK)
False

Note

For the majority of new code, Enum and Flag are strongly recommended, since IntEnum and IntFlag break some semantic promises of an enumeration (by being comparable to integers, and thus by transitivity to other unrelated enumerations). IntEnum and IntFlag should be used only in cases where Enum and Flag will not do; for example, when integer constants are replaced with enumerations, or for interoperability with other systems.

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:

When subclassing Enum, mix-in types must appear before Enum itself in the sequence of bases, as in the IntEnum example above.
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 type.
When another data type is mixed in, the value attribute is not the same as the enum member itself, although it is equivalent and will compare equal.
%-style formatting: %s and %r call the Enum class’s __str__() and __repr__() respectively; other codes (such as %i or %h for IntEnum) treat the enum member as its mixed-in type.
Formatted string literals, str.format(), and format() will use the mixed-in type’s __format__() unless __str__() or __format__() is overridden in the subclass, in which case the overridden methods or Enum methods will be used. Use the !s and !r format codes to force usage of the Enum class’s __str__() and __repr__() methods.

When to use `new()` vs. `init()`¶

__new__() must be used whenever you want to customize the actual value of the Enum member. Any other modifications may go in either __new__() or __init__(), with __init__() being preferred.

For example, if you want to pass several items to the constructor, but only want one of them to be the value:

>>> class Coordinate(bytes, Enum):
...     """
...     Coordinate with binary codes that can be indexed by the int code.
...     """
...     def __new__(cls, value, label, unit):
...         obj = bytes.__new__(cls, [value])
...         obj._value_ = value
...         obj.label = label
...         obj.unit = unit
...         return obj
...     PX = (0, 'P.X', 'km')
...     PY = (1, 'P.Y', 'km')
...     VX = (2, 'V.X', 'km/s')
...     VY = (3, 'V.Y', 'km/s')
...

>>> print(Coordinate['PY'])
Coordinate.PY

>>> print(Coordinate(3))
Coordinate.VY

Interesting examples¶

While Enum, IntEnum, IntFlag, and Flag 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.

Omitting values¶

In many use-cases one doesn’t care what the actual value of an enumeration is. There are several ways to define this type of simple enumeration:

use instances of auto for the value
use instances of object as the value
use a descriptive string as the value
use a tuple as the value and a custom __new__() to replace the tuple with an int value

Using any of these methods signifies to the user that these values are not important, and also enables one to add, remove, or reorder members without having to renumber the remaining members.

Whichever method you choose, you should provide a repr() that also hides the (unimportant) value:

>>> class NoValue(Enum):
...     def __repr__(self):
...         return '<%s.%s>' % (self.__class__.__name__, self.name)
...

Using `auto`¶

Using auto would look like:

>>> class Color(NoValue):
...     RED = auto()
...     BLUE = auto()
...     GREEN = auto()
...
>>> Color.GREEN
<Color.GREEN>

Using `object`¶

Using object would look like:

>>> class Color(NoValue):
...     RED = object()
...     GREEN = object()
...     BLUE = object()
...
>>> Color.GREEN
<Color.GREEN>

Using a descriptive string¶

Using a string as the value would look like:

>>> class Color(NoValue):
...     RED = 'stop'
...     GREEN = 'go'
...     BLUE = 'too fast!'
...
>>> Color.GREEN
<Color.GREEN>
>>> Color.GREEN.value
'go'

Using a custom `new()`¶

Using an auto-numbering __new__() would look like:

>>> class AutoNumber(NoValue):
...     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
<Color.GREEN>
>>> Color.GREEN.value
2

To make a more general purpose AutoNumber, add *args to the signature:

>>> class AutoNumber(NoValue):
...     def __new__(cls, *args):      # this is the only change from above
...         value = len(cls.__members__) + 1
...         obj = object.__new__(cls)
...         obj._value_ = value
...         return obj
...

Then when you inherit from AutoNumber you can write your own __init__ to handle any extra arguments:

>>> class Swatch(AutoNumber):
...     def __init__(self, pantone='unknown'):
...         self.pantone = pantone
...     AUBURN = '3497'
...     SEA_GREEN = '1246'
...     BLEACHED_CORAL = () # New color, no Pantone code yet!
...
>>> Swatch.SEA_GREEN
<Swatch.SEA_GREEN>
>>> Swatch.SEA_GREEN.pantone
'1246'
>>> Swatch.BLEACHED_CORAL.pantone
'unknown'

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.

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

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.

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

TimePeriod¶

An example to show the _ignore_ attribute in use:

>>> from datetime import timedelta
>>> class Period(timedelta, Enum):
...     "different lengths of time"
...     _ignore_ = 'Period i'
...     Period = vars()
...     for i in range(367):
...         Period['day_%d' % i] = i
...
>>> list(Period)[:2]
[<Period.day_0: datetime.timedelta(0)>, <Period.day_1: datetime.timedelta(days=1)>]
>>> list(Period)[-2:]
[<Period.day_365: datetime.timedelta(days=365)>, <Period.day_366: datetime.timedelta(days=366)>]

How are Enums different?¶

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

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_enum_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__()).

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.

Finer Points¶

Supported `dunder` names¶

__members__ is a read-only ordered mapping of member_name:member items. It is only available on the class.

__new__(), if specified, must create and return the enum members; it is also a very good idea to set the member’s _value_ appropriately. Once all the members are created it is no longer used.

Supported `_sunder_` names¶

_name_ – name of the member
_value_ – value of the member; can be set / modified in __new__
_missing_ – a lookup function used when a value is not found; may be overridden
_ignore_ – a list of names, either as a list or a str, that will not be transformed into members, and will be removed from the final class
_order_ – used in Python 2/3 code to ensure member order is consistent (class attribute, removed during class creation)
_generate_next_value_ – used by the Functional API and by auto to get an appropriate value for an enum member; may be overridden

New in version 3.6: _missing_, _order_, _generate_next_value_

New in version 3.7: _ignore_

To help keep Python 2 / Python 3 code in sync an _order_ attribute can be provided. It will be checked against the actual order of the enumeration and raise an error if the two do not match:

>>> class Color(Enum):
...     _order_ = 'RED GREEN BLUE'
...     RED = 1
...     BLUE = 3
...     GREEN = 2
...
Traceback (most recent call last):
...
TypeError: member order does not match _order_

Note

In Python 2 code the _order_ attribute is necessary as definition order is lost before it can be recorded.

_Private__names¶

Private names will be normal attributes in Python 3.11 instead of either an error or a member (depending on if the name ends with an underscore). Using these names in 3.10 will issue a DeprecationWarning.

`Enum` member type¶

Enum members are instances of their Enum class, and are normally accessed as EnumClass.member. Under certain circumstances they can also be accessed as EnumClass.member.member, but you should never do this as that lookup may fail or, worse, return something besides the Enum member you are looking for (this is another good reason to use all-uppercase names for members):

>>> class FieldTypes(Enum):
...     name = 0
...     value = 1
...     size = 2
...
>>> FieldTypes.value.size
<FieldTypes.size: 2>
>>> FieldTypes.size.value
2

Note

This behavior is deprecated and will be removed in 3.11.

Changed in version 3.5.

Boolean value of `Enum` classes and members¶

Enum members that are mixed with non-Enum types (such as int, str, etc.) are evaluated according to the mixed-in type’s rules; otherwise, all members evaluate as True. To make your own Enum’s boolean evaluation depend on the member’s value add the following to your class:

def __bool__(self):
    return bool(self.value)

Enum classes always evaluate as True.

`Enum` classes with methods¶

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__', 'mass', 'name', 'radius', 'surface_gravity', 'value']

Combining members of `Flag`¶

If a combination of Flag members is not named, the repr() will include all named flags and all named combinations of flags that are in the value:

>>> class Color(Flag):
...     RED = auto()
...     GREEN = auto()
...     BLUE = auto()
...     MAGENTA = RED | BLUE
...     YELLOW = RED | GREEN
...     CYAN = GREEN | BLUE
...
>>> Color(3)  # named combination
<Color.YELLOW: 3>
>>> Color(7)      # not named combination
<Color.CYAN|MAGENTA|BLUE|YELLOW|GREEN|RED: 7>

Note

In 3.11 unnamed combinations of flags will only produce the canonical flag members (aka single-value flags). So Color(7) will produce something like <Color.BLUE|GREEN|RED: 7>.

enum — Support for enumerations¶

Module Contents¶

Creating an Enum¶

Programmatic access to enumeration members and their attributes¶

Duplicating enum members and values¶

Ensuring unique enumeration values¶

Using automatic values¶

Iteration¶

Comparisons¶

Allowed members and attributes of enumerations¶

Restricted Enum subclassing¶

Pickling¶

Functional API¶

Derived Enumerations¶

IntEnum¶

IntFlag¶

Flag¶

Others¶

When to use __new__() vs. __init__()¶

Interesting examples¶

Omitting values¶

Using auto¶

Using object¶

Using a descriptive string¶

Using a custom __new__()¶

OrderedEnum¶

DuplicateFreeEnum¶

Planet¶

TimePeriod¶

How are Enums different?¶

Enum Classes¶

Enum Members (aka instances)¶

Finer Points¶

Supported __dunder__ names¶

Supported _sunder_ names¶

_Private__names¶

Enum member type¶

Boolean value of Enum classes and members¶

Enum classes with methods¶

Combining members of Flag¶

`enum` — Support for enumerations¶

When to use `new()` vs. `init()`¶

Using `auto`¶

Using `object`¶

Using a custom `new()`¶

Supported `dunder` names¶

Supported `_sunder_` names¶

`Enum` member type¶

Boolean value of `Enum` classes and members¶

`Enum` classes with methods¶

Combining members of `Flag`¶