Python: Enum枚举的实现

      从C系语言过来用Python，好不容易适应了写代码不打花括号，突然有一天发现它居然木有枚举……于是stackoverflow了一把，发现神人的枚举（enum）实现到处都是，于是汉化总结过来。

如果是新版Python用户（Python 3.4 with PEP 435）：

from enum import Enum
Animal = Enum('Animal', 'ant bee cat dog')

or

class Animals(Enum):
    ant = 1
    bee = 2
    cat = 3
    dog = 4

旧版Python用户可以充分发挥动态语言的优越性来构造枚举，有简单的：

def enum(**enums):
    return type('Enum', (), enums)

Numbers = enum(ONE=1, TWO=2, THREE='three')
# Numbers.ONE == 1, Numbers.TWO == 2 and Numbers.THREE == 'three'

有复杂的：

def enum(*sequential, **named):
    enums = dict(zip(sequential, range(len(sequential))), **named)
    return type('Enum', (), enums)

Numbers = enum('ZERO', 'ONE', 'TWO')
# Numbers.ZERO == 0 and Numbers.ONE == 1

有带值到名称映射的：

def enum(*sequential, **named):
    enums = dict(zip(sequential, range(len(sequential))), **named)
    reverse = dict((value, key) for key, value in enums.iteritems())
    enums['reverse_mapping'] = reverse
    return type('Enum', (), enums)

# Numbers.reverse_mapping['three'] == 'THREE'

有用set实现的：

class Enum(set):
    def __getattr__(self, name):
        if name in self:
            return name
        raise AttributeError

Animals = Enum(["DOG", "CAT", "HORSE"])
print Animals.DOG

有用range实现的：

dog, cat, rabbit = range(3)

# or

class Stationary:
    (Pen, Pencil, Eraser) = range(0, 3)
print Stationary.Pen

有用tuple实现的：

class Enum(tuple): __getattr__ = tuple.index

State = Enum(['Unclaimed', 'Claimed'])
print State.Claimed

有用namedtuple实现的：

from collections import namedtuple

def enum(*keys):
    return namedtuple('Enum', keys)(*keys)

MyEnum = enum('FOO', 'BAR', 'BAZ')

# 带字符数字映射的，像C/C++
def enum(*keys):
    return namedtuple('Enum', keys)(*range(len(keys)))

# 带字典映射的，可以映射出各种类型，不局限于数字
def enum(**kwargs):
    return namedtuple('Enum', kwargs.keys())(*kwargs.values())