当需要定义常量时，一个办法是用大写变量通过整数来定义
好处是简单，缺点是类型是int，并且仍然是变量
Enum可以把一组相关常量定义在一个class中，且class不可变，而且成员可以直接比较，这样这些相关常量就不会变化了
使用枚举类
from enum import Enum
Month = Enum('Month', ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'))
for name, member in Month.__members__.items():
print(name, '=>', member, ',', member.value) #value属性则是自动赋给成员的int常量，默认从1开始计数
#输出：Jan => Month.Jan , 1
#输出：Feb => Month.Feb , 2
#输出：Mar => Month.Mar , 3
#输出：Apr => Month.Apr , 4
#输出：May => Month.May , 5
#输出：Jun => Month.Jun , 6
#输出：Jul => Month.Jul , 7
#输出：Aug => Month.Aug , 8
#输出：Sep => Month.Sep , 9
#输出：Oct => Month.Oct , 10
#输出：Nov => Month.Nov , 11
#输出：Dec => Month.Dec , 12
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
from enum import Enum, unique
@unique #@unique装饰器可以帮助我们检查保证没有重复值
class Weekday(Enum):
Sun = 00
Mon = 01
Tue = 02
Wed = 03
Thu = 04
Fri = 05
Sat = 06
day1 = Weekday.Mon
print(day1) #输出：Weekday.Mon
print(day1.value) #输出：01，value属性是枚举常量的值
print(Weekday.Tue) #输出：Weekday.Tue
print(Weekday['Tue']) #输出：Weekday.Tue，可用成员名称引用枚举常量
print(Weekday['Tue'].value) #输出：02
print(day1 == Weekday.Mon) #输出：True
print(day1 == Weekday.Tue) #输出：False
print(Weekday('03')) #输出：Weekday.Wed，根据value值获取枚举常量
print(Weekday('03').value) #输出：03，根据value值获取枚举常量
print(day1 == Weekday('01')) #输出：True
Weekday(7) #报错
for name, member in Weekday.__members__.items():
print(name, '=>', member)
#输出：Sun => Weekday.Sun
#输出：Mon => Weekday.Mon
#输出：Tue => Weekday.Tue
#输出：Wed => Weekday.Wed
#输出：Thu => Weekday.Thu
#输出：Fri => Weekday.Fri
#输出：Sat => Weekday.Sat