python——使用枚舉類

當我們需要定義常量時，一個辦法是用大寫變量通過整數來定義，例如月份：

JAN = 1
FEB = 2
MAR = 3
...
NOV = 11
DEC = 12

好處是簡單，缺點是類型是int，並且仍然是變量。

更好的方法是爲這樣的枚舉類型定義一個class類型，然後，每個常量都是class的一個唯一實例。Python提供了Enum類來實現這個功能：

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屬性則是自動賦給成員的int常量，默認從1開始計數。

如果需要更精確地控制枚舉類型，可以從Enum派生出自定義類：

from enum import Enum, unique
@unique
class Weekday(Enum):
    Sun = 0 # Sun的value被設定爲0
    Mon = 1
    Tue = 2
    Wed = 3
    Thu = 4
    Fri = 5
    Sat = 6
#調用枚舉成員的 3 種方式
>>>print(print(Weekday.Tue))
Weekday.Tue
>>>print(Weekday['Tue'])
Weekday.Tue
>>>print(Weekday(1))
Weekday.Mon

#調取枚舉成員中的 value 和 name
>>>print(Weekday.Mon.value)
1
>>>print(Weekday.Mon.name)
Mon

#遍歷枚舉類中所有成員的2種方式
>>> for name, member in Weekday.__members__.items():
...     print(name, '=>', member)
...
Sun => Weekday.Sun
Mon => Weekday.Mon
Tue => Weekday.Tue
Wed => Weekday.Wed
Thu => Weekday.Thu
Fri => Weekday.Fri
Sat => Weekday.Sat

>>>for weekday in Weekday:
...    print(weekday)
Weekday.Sun
Weekday.Mon
Weekday.Tue
Weekday.Wed
Weekday.Thu
Weekday.Fri
Weekday.Sat

@unique裝飾器可以幫助我們檢查保證沒有重複值。

枚舉類成員之間不能比較打下，但可以用 == 或者 is 進行比較是否相等