python学习笔记，特做记录，分享给大家，希望对大家有所帮助。
使用枚举类
当我们需要定义常量时，一个办法是用大写变量通过整数来定义，例如月份：
JAN = 1
FEB = 2
MAR = 3
...
NOV = 11
DEC = 12
好处是简单，缺点是类型是int，并且仍然是变量。
更好的方法是为这样的枚举类型定义一个class类型，然后，每个常量都是class的一个唯一实例。Python提供了Enum类来实现这个功能：
from enum import Enum
Month = Enum('Month', ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'))
这样我们就获得了Month类型的枚举类，可以直接使用Month.Jan来引用一个常量，或者枚举它的所有成员：
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。
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
class Weekday(Enum):
Sun = 0 # Sun的value被设定为0
Mon = 1
Tue = 2
Wed = 3
Thu = 4
Fri = 5
Sat = 6
@unique装饰器可以帮助我们检查保证没有重复值。
访问这些枚举类型可以有若干种方法：
day1 = Weekday.Mon
print day1
print Weekday.Tue
print Weekday['Tue']
print Weekday.Tue.value
print day1 == Weekday.Mon
print day1 == Weekday.Tue
print Weekday(1)
print day1 == Weekday(1)
Weekday(7)
for name, member in Weekday.__members__.items():
print name, '=>', member
运行结果：
Weekday.Mon
Weekday.Tue
Weekday.Tue
2
True
False
Weekday.Mon
True
Traceback (most recent call last):
...
ValueError: 7 is not a valid Weekday
Sun => Weekday.Sun
Mon => Weekday.Mon
Tue => Weekday.Tue
Wed => Weekday.Wed
Thu => Weekday.Thu
Fri => Weekday.Fri
Sat => Weekday.Sat
Process finished with exit code 0
可见，既可以用成员名称引用枚举常量，又可以直接根据value的值获得枚举常量。
