我主要是c#开发人员,但我目前正在用Python开发一个项目。
我如何在Python中表示等价的Enum ?
我主要是c#开发人员,但我目前正在用Python开发一个项目。
我如何在Python中表示等价的Enum ?
当前回答
这是我见过的最好的一个:“Python中的First Class Enums”
http://code.activestate.com/recipes/413486/
It gives you a class, and the class contains all the enums. The enums can be compared to each other, but don't have any particular value; you can't use them as an integer value. (I resisted this at first because I am used to C enums, which are integer values. But if you can't use it as an integer, you can't use it as an integer by mistake so overall I think it is a win.) Each enum is a unique value. You can print enums, you can iterate over them, you can test that an enum value is "in" the enum. It's pretty complete and slick.
编辑(cfi):上面的链接不兼容Python 3。下面是我将枚举.py移植到Python 3的端口:
def cmp(a,b):
if a < b: return -1
if b < a: return 1
return 0
def Enum(*names):
##assert names, "Empty enums are not supported" # <- Don't like empty enums? Uncomment!
class EnumClass(object):
__slots__ = names
def __iter__(self): return iter(constants)
def __len__(self): return len(constants)
def __getitem__(self, i): return constants[i]
def __repr__(self): return 'Enum' + str(names)
def __str__(self): return 'enum ' + str(constants)
class EnumValue(object):
__slots__ = ('__value')
def __init__(self, value): self.__value = value
Value = property(lambda self: self.__value)
EnumType = property(lambda self: EnumType)
def __hash__(self): return hash(self.__value)
def __cmp__(self, other):
# C fans might want to remove the following assertion
# to make all enums comparable by ordinal value {;))
assert self.EnumType is other.EnumType, "Only values from the same enum are comparable"
return cmp(self.__value, other.__value)
def __lt__(self, other): return self.__cmp__(other) < 0
def __eq__(self, other): return self.__cmp__(other) == 0
def __invert__(self): return constants[maximum - self.__value]
def __nonzero__(self): return bool(self.__value)
def __repr__(self): return str(names[self.__value])
maximum = len(names) - 1
constants = [None] * len(names)
for i, each in enumerate(names):
val = EnumValue(i)
setattr(EnumClass, each, val)
constants[i] = val
constants = tuple(constants)
EnumType = EnumClass()
return EnumType
if __name__ == '__main__':
print( '\n*** Enum Demo ***')
print( '--- Days of week ---')
Days = Enum('Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa', 'Su')
print( Days)
print( Days.Mo)
print( Days.Fr)
print( Days.Mo < Days.Fr)
print( list(Days))
for each in Days:
print( 'Day:', each)
print( '--- Yes/No ---')
Confirmation = Enum('No', 'Yes')
answer = Confirmation.No
print( 'Your answer is not', ~answer)
其他回答
为了解码二进制文件格式,我曾经需要一个Enum类。我碰巧想要的特性是简洁的枚举定义,通过整数值或字符串自由创建枚举实例的能力,以及有用的表示。这是我最后得出的结论:
>>> class Enum(int):
... def __new__(cls, value):
... if isinstance(value, str):
... return getattr(cls, value)
... elif isinstance(value, int):
... return cls.__index[value]
... def __str__(self): return self.__name
... def __repr__(self): return "%s.%s" % (type(self).__name__, self.__name)
... class __metaclass__(type):
... def __new__(mcls, name, bases, attrs):
... attrs['__slots__'] = ['_Enum__name']
... cls = type.__new__(mcls, name, bases, attrs)
... cls._Enum__index = _index = {}
... for base in reversed(bases):
... if hasattr(base, '_Enum__index'):
... _index.update(base._Enum__index)
... # create all of the instances of the new class
... for attr in attrs.keys():
... value = attrs[attr]
... if isinstance(value, int):
... evalue = int.__new__(cls, value)
... evalue._Enum__name = attr
... _index[value] = evalue
... setattr(cls, attr, evalue)
... return cls
...
使用它的一个异想天开的例子:
>>> class Citrus(Enum):
... Lemon = 1
... Lime = 2
...
>>> Citrus.Lemon
Citrus.Lemon
>>>
>>> Citrus(1)
Citrus.Lemon
>>> Citrus(5)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __new__
KeyError: 5
>>> class Fruit(Citrus):
... Apple = 3
... Banana = 4
...
>>> Fruit.Apple
Fruit.Apple
>>> Fruit.Lemon
Citrus.Lemon
>>> Fruit(1)
Citrus.Lemon
>>> Fruit(3)
Fruit.Apple
>>> "%d %s %r" % ((Fruit.Apple,)*3)
'3 Apple Fruit.Apple'
>>> Fruit(1) is Citrus.Lemon
True
主要特点:
str(), int()和repr()都会产生最有用的输出,分别是枚举的名称,它的整数值,以及返回枚举的Python表达式。 构造函数返回的枚举值严格限制为预定义值,没有意外的枚举值。 枚举值是单例的;它们可以与之严格比较
这里是一个实现:
class Enum(set):
def __getattr__(self, name):
if name in self:
return name
raise AttributeError
下面是它的用法:
Animals = Enum(["DOG", "CAT", "HORSE"])
print(Animals.DOG)
在Java pre-JDK 5中使用的类型安全枚举模式有一个 优势的数量。就像Alexandru的回答一样,你创建了一个 类和类级别字段是枚举值;然而,枚举 值是类的实例,而不是小整数。这已经 优点是枚举值不会在不经意间比较相等 对于小整数,你可以控制它们的打印方式,任意添加 方法(如果有用的话),并使用isinstance进行断言:
class Animal:
def __init__(self, name):
self.name = name
def __str__(self):
return self.name
def __repr__(self):
return "<Animal: %s>" % self
Animal.DOG = Animal("dog")
Animal.CAT = Animal("cat")
>>> x = Animal.DOG
>>> x
<Animal: dog>
>>> x == 1
False
python-dev上最近的一个线程指出,在野外有几个枚举库,包括:
flufl.enum lazr.enum ... 和富有想象力的enum
虽然最初的全会提案PEP 354在几年前被否决,但它不断被提出。本打算将某种enum添加到3.2,但它被推回到3.3,然后被遗忘。现在有一个PEP 435打算包含在Python 3.4中。PEP 435的参考实现是flufl.enum。
截至2013年4月,似乎有一个普遍的共识,即应该在3.4的标准库中添加一些东西——只要人们能够就“一些东西”应该是什么达成一致。这是最难的部分。看看从这里和这里开始的线程,以及2013年初的六个其他线程。
与此同时,每次出现这个问题时,PyPI、ActiveState等上都会出现大量的新设计和实现,所以如果你不喜欢FLUFL设计,可以尝试一下PyPI搜索。
我用什么:
class Enum(object):
def __init__(self, names, separator=None):
self.names = names.split(separator)
for value, name in enumerate(self.names):
setattr(self, name.upper(), value)
def tuples(self):
return tuple(enumerate(self.names))
使用方法:
>>> state = Enum('draft published retracted')
>>> state.DRAFT
0
>>> state.RETRACTED
2
>>> state.FOO
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
AttributeError: 'Enum' object has no attribute 'FOO'
>>> state.tuples()
((0, 'draft'), (1, 'published'), (2, 'retracted'))
这就给出了整数常数,比如状态。PUBLISHED和在Django模型中用作选项的二元组。