第二章：数据结构-enum:枚举类型-唯一枚举值

2.1.4 唯一枚举值
有相同值的Enum成员会被处理为同一个成员对象的别名引用。别名可以避免Enum的迭代器中出现重复的值。

import enum

class BugStatus(enum.Enum):

    new = 7
    incomplete = 6
    invalid = 5
    wont_fix = 4
    in_progress = 3
    fix_committed = 2
    fix_released = 1

    by_design = 4
    closed = 1

for status in BugStatus:
    print('{:15} = {}'.format(status.name,status.value))

print('\nSame:by_design is wont_fix: ',
      BugStatus.by_design is BugStatus.wont_fix)

print('Same:closed is fix_released: ',
      BugStatus.closed is BugStatus.fix_released)

由于by_design和closed是其他成员的别名，迭代处理Enum时它们不会单独出现在输出中。一个成员的规范名是与这个值关联的第一个名字。
运行结果：

如果要求所有成员有唯一的值，则要为Enum增加@unique修饰符。

import enum

@enum.unique
class BugStatus(enum.Enum):

    new = 7
    incomplete = 6
    invalid = 5
    wont_fix = 4
    in_progress = 3
    fix_committed = 2
    fix_released = 1

    # This will trigger an error with unique applied.
    by_design = 4
    closed = 1

解释Enum类时，有重复值的成员会触发一个ValueError异常。
运行结果：