enum
— 支持枚举
¶
3.4 版新增。
源代码： Lib/enum.py
枚举是绑定到唯一常量值的一组符号名称 (成员)。在枚举中，成员可以通过身份进行比较，且枚举本身还可以迭代。
This module defines two enumeration classes that can be used to define unique sets of names and values:
Enum
and
IntEnum
。它还定义了一个装饰器
unique()
.
枚举。
unique
(
)
¶
Enum class decorator that ensures only one name is bound to any one value.
枚举的创建是使用
class
syntax, which makes them easy to read and write. An alternative creation method is described in
函数式 API
。要定义枚举，子类
Enum
如下：
>>> from enum import Enum >>> class Color(Enum): ... red = 1 ... green = 2 ... blue = 3 ...
注意
Nomenclature
Color
是
enumeration
(或
enum
)
Color.red
,
Color.green
，等，是
enumeration 成员
(或
enum 成员
).
Color.red
is
red
，值对于
Color.blue
is
3
，等)
注意
即使使用
class
句法能创建 Enum, 但 Enum 不是正常 Python 类。见
枚举有什么不同？
了解更多细节。
enumeration 成员拥有人类可读的字符串表示：
>>> print(Color.red) Color.red
...while their
repr
拥有更多信息：
>>> 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 >>>
枚举成员还有恰好包含其项名称的特性：
>>> print(Color.red.name) red
枚举支持迭代，按定义次序：
>>> 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
枚举成员可哈希，因此它们可用于字典和集：
>>> apples = {} >>> apples[Color.red] = 'red delicious' >>> apples[Color.green] = 'granny smith' >>> apples == {Color.red: 'red delicious', Color.green: 'granny smith'} True
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>
若想要访问枚举成员通过 name ，使用项访问：
>>> Color['red'] <Color.red: 1> >>> Color['green'] <Color.green: 2>
若有枚举成员且需要其
name
or
value
:
>>> member = Color.red >>> member.name 'red' >>> member.value 1
拥有 2 同名成员的枚举是无效的：
>>> 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>
注意
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.
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:
@
枚举。
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
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']
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
The examples above use integers for enumeration values. Using integers is short and handy (and provided by default by the 函数式 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.) 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
范例。
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.)
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.
注意
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.
The
Enum
类可调用，提供以下 API 功能：
>>> 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
相当于：
>>> 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
若
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)
| 值： |
What the new Enum class will record as its name. |
|---|---|
| 名称： |
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' 或名称迭代器： ['red', 'green', 'blue'] 或 (name, value) 对迭代器： [('cyan', 4), ('magenta', 5), ('yellow', 6)] 或映射： {'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： |
能混入新 Enum 类的类型。 |
| start: |
number to start counting at if only names are passed in. |
3.5 版改变： The start 参数被添加。
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
枚举：
>>> 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.
While
IntEnum
属于
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
而不是
int
.
一些规则：
Enum
, mix-in types must appear before
Enum
itself in the sequence of bases, as in the
IntEnum
example above.
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
.
value
属性为
not the
same
as the enum member itself, although it is equivalent and will compare
equal.
Enum
类的
__str__()
and
__repr__()
respectively; other codes (such as
%i
or
%h
for IntEnum) treat the enum member as its mixed-in type.
str.format()
(或
format()
) will use the mixed-in
type’s
__format__()
。若
Enum
类的
str()
or
repr()
is desired, use the
!s
or
!r
format codes.
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.
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
注意
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.
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
引发错误，若发现成员名称重复而不是创建别名：
>>> 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'
注意
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.
若
__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
Enums have a custom metaclass that affects many aspects of both derived Enum classes and their instances (members).
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__()
).
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.
Enum
members are instances of an
Enum
class, and even though they are accessible as
EnumClass.member
, they should not be accessed directly from the member as that lookup may fail or, worse, return something besides the
Enum
member you looking for:
>>> class FieldTypes(Enum): ... name = 0 ... value = 1 ... size = 2 ... >>> FieldTypes.value.size <FieldTypes.size: 2> >>> FieldTypes.size.value 2
3.5 版改变。
The
__members__
attribute is only available on the class.
若给出
Enum
子类额外方法，像
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']
The
__new__()
method will only be used for the creation of the
Enum
members – after that it is replaced. Any custom
__new__()
method must create the object and set the
_value_
attribute appropriately.
If you wish to change how
Enum
members are looked up you should either write a helper function or a
classmethod()
为
Enum
子类。
enum
— 支持枚举