内置枚举类Enum
使用内置的枚举类Enum，需要引入enum模块。
例如：
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from enum import Enum
#Enum函数新建枚举Month
Month = Enum('Month', ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'))
#枚举Month的类型
print(type(Month))
#循环打印枚举
for name, member in Month.__members__.items():
print(name, '=>', member, ',', member.value)
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执行以上程序会输出如下结果：
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<class 'enum.EnumMeta'>
Jan => Month.Jan , 1
Feb => Month.Feb , 2
Mar => Month.Mar , 3
Apr => Month.Apr , 4
May => Month.May , 5
Jun => Month.Jun , 6
Jul => Month.Jul , 7
Aug => Month.Aug , 8
Sep => Month.Sep , 9
Oct => Month.Oct , 10
Nov => Month.Nov , 11
Dec => Month.Dec , 12
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value属性则是自动赋给成员的int常量，默认从1开始计数。
自定义枚举类
如果需要更精确地控制枚举类型，可以从Enum派生出自定义类：
1.枚举的定义
枚举定义用class关键字，继承Enum类。
例如：
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from enum import Enum
class Weekday(Enum):
Sun = 0
Mon = 1
Tue = 2
Wed = 3
Thu = 4
Fri = 5
Sat = 6
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代码分析：
上面的代码，我们定义了枚举Weekday。Weekday枚举有7个成员，分别是Weekday.Sun、Weekday.Mon等。每一个成员都有它们各自名称和值，Weekday.Sun成员的名称是：Sun，值是：0。每个成员的数据类型就是它所属的枚举。
1.1 定义枚举时，成员名称不允许重复
例如：
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from enum import Enum
class Weekday(Enum):
Mon = 1
Mon = 2
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上面的代码，就无法执行。提示错误：
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Traceback (most recent call last):
File "main.py", line 2, in <module>
class Weekday(Enum):
File "main.py", line 4, in Weekday
Mon = 2
File "/usr/lib64/python3.6/enum.py", line 92, in __setitem__
raise TypeError('Attempted to reuse key: %r' % key)
TypeError: Attempted to reuse key: 'Mon'
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1.2 默认情况下，不同的成员值允许相同。但是两个相同值的成员，第二个成员的名称被视作第一个成员的别名
例如：
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from enum import Enum
class Weekday(Enum):
Mon = 1
Mon_alias = 1
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Weekday.Mon_alias的名称Mon_alias就被视作成员Weekday.Mon名称Mon的别名。
1.3 如果枚举中存在相同值的成员，在通过值获取枚举成员时，只能获取到第一个成员
例如：
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from enum import Enum
class Weekday(Enum):
Mon = 1
Mon_alias = 1
print(Weekday(1))
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输出结果为：Weekday.Mon
1.4 如果要限制定义枚举时，不能定义相同值的成员。可以使用装饰器@unique
例如：
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from enum import Enum, unique
@unique
class Weekday(Enum):
Mon = 1
Mon_alias = 1
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再执行就会提示错误：
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Traceback (most recent call last):
File "main.py", line 3, in <module>
class Weekday(Enum):
File "/usr/lib64/python3.6/enum.py", line 834, in unique
(enumeration, alias_details))
ValueError: duplicate values found in <enum 'Weekday'>: Mon_alias -> Mon
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2.枚举取值
通过成员的名称来获取成员
Weekday[‘Mon’]
通过成员值来获取成员
Weekday(1)
通过成员，来获取它的名称和值
Mon_member = Weekday.Mon
Mon_member.name
Mon_member.value
3.迭代器
3.1 枚举支持迭代器，可以遍历枚举成员
例如：
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from enum import Enum
class Weekday(Enum):
Sun = 0
Mon = 1
Tue = 2
Wed = 3
Thu = 4
Fri = 5
Sat = 6
for day in Weekday:
print(day)
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输出结果是，枚举的所有成员。
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Weekday.Sun
Weekday.Mon
Weekday.Tue
Weekday.Wed
Weekday.Thu
Weekday.Fri
Weekday.Sat
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3.2 如果枚举有值重复的成员，循环遍历枚举时只获取值重复成员的第一个成员
例如：
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from enum import Enum
class Weekday(Enum):
Sun = 0
Mon = 1
Tue = 2
Wed = 3
Thu = 4
Fri = 5
Sat = 6
Mon_alias = 1
for day in Weekday:
print(day)
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输出结果是：
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Weekday.Sun
Weekday.Mon
Weekday.Tue
Weekday.Wed
Weekday.Thu
Weekday.Fri
Weekday.Sat
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但是Weekday.Mon_alias并没有出现在输出结果中。
3.3 如果想把值重复的成员也遍历出来，要用枚举的一个特殊属性members
例如：
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from enum import Enum
class Weekday(Enum):
Sun = 0
Mon = 1
Tue = 2
Wed = 3
Thu = 4
Fri = 5
Sat = 6
Mon_alias = 1
for day in Weekday:
print(day)
for day in Weekday.__members__.items():
print(day)
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输出结果：
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Weekday.Sun
Weekday.Mon
Weekday.Tue
Weekday.Wed
Weekday.Thu
Weekday.Fri
Weekday.Sat
('Sun', <Weekday.Sun: 0>)
('Mon', <Weekday.Mon: 1>)
('Tue', <Weekday.Tue: 2>)
('Wed', <Weekday.Wed: 3>)
('Thu', <Weekday.Thu: 4>)
('Fri', <Weekday.Fri: 5>)
('Sat', <Weekday.Sat: 6>)
('Mon_alias', <Weekday.Mon: 1>)
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4.枚举比较
4.1 枚举成员可进行同一性比较
Weekday.Mon is Weekday.Mon
输出结果是：True
Weekday.Mon is not Weekday.Sat
输出结果是：True
4.2 枚举成员可进行等值比较
Weekday.Mon == Weekday.Sat
输出结果是：False
Weekday.Mon != Weekday.Sat
输出结果是：True
4.3 枚举成员不能进行大小比较
Weekday.Mon < Weekday.Sat
输出结果出错：TypeError: unorderable types: Weekday() < Weekday()
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