enum --- 对枚举的支持

3.4 新版功能.

源代码: Lib/enum.py


枚举是一组符号名称(枚举成员)的集合,枚举成员应该是唯一的、不可变的。在枚举中,可以对成员进行恒等比较,并且枚举本身是可迭代的。

模块内容

此模块定义了四个枚举类,它们可被用来定义名称和值的不重复集合: Enum, IntEnum, FlagIntFlag。 此外还定义了一个装饰器 unique() 和一个辅助类 auto

class enum.Enum

此基类用于创建枚举常量。 请参阅 Functional API 小节了解另一种替代性的构建语法。

class enum.IntEnum

此基类用于创建属于 int 的子类的枚举常量。

class enum.IntFlag

此基类用于创建可使用按位运算符进行组合而不会丢失其 IntFlag 成员资格的枚举常量。 IntFlag 成员同样也是 int 的子类。

class enum.Flag

此基类用于创建枚举常量 可使用按位运算符进行组合而不会丢失其 Flag 成员资格的枚举常量。

enum.unique()

此 Enum 类装饰器可确保只将一个名称绑定到任意一个值。

class enum.auto

实例会被替换为一个可用作 Enum 成员的正确值。

3.6 新版功能: Flag, IntFlag, auto

创建一个 Enum

枚举是使用 class 语法来创建的,这使得它们易于读写。 另一种替代创建方法的描述见 Functional API。 要定义一个枚举,可以对 Enum 进行如下的子类化:

>>> from enum import Enum
>>> class Color(Enum):
...     RED = 1
...     GREEN = 2
...     BLUE = 3
...

注解

Enum 的成员值

成员值可以为任意类型: int, str 等等。 如果具体的值不重要,你可以使用 auto 实例,将为你选择适当的值。 但如果你混用 auto 与其他值则需要小心谨慎。

注解

命名法

  • Color 是一个 枚举 (或称 enum)

  • 属性 Color.RED, Color.GREEN 等等是 枚举成员 (或称 enum 成员) 并且被用作常量。

  • 枚举成员具有 名称 (Color.RED 的名称为 REDColor.BLUE 的值为 3 等等。)

注解

虽然我们使用 class 语法来创建 Enum,但 Enum 并不是普通的 Python 类。 更多细节请参阅 How are Enums different?

枚举成员具有适合人类阅读的表示形式:

>>> print(Color.RED)
Color.RED

...而它们的 repr 包含更多信息:

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

一个枚举成员的 type 就是它所从属的枚举:

>>> 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>

注解

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'>]

注解

The goal of the default _generate_next_value_() methods 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.

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']

比较

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 whatever value(s) were 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: 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 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.

注解

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__)

警告

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)

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}
模块

name of module where new Enum class can be found.

qualname

where in module new Enum class can be found.

类型

type to mix in to new Enum class.

start

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

在 3.5 版更改: 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.

3.6 新版功能.

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>

标志

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.

3.6 新版功能.

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

注解

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:

  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 equivalent and will compare equal.

  4. %-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.

  5. Formatted string literals, str.format(), and format() will use the mixed-in type's __format__(). If the Enum class's str() or repr() is desired, use the !s or !r format codes.

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

注解

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'

注解

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

3.6 新版功能: _missing_, _order_, _generate_next_value_

3.7 新版功能: _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_

注解

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

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

在 3.5 版更改.

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__', 'name', '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>