Python——枚举类和元类

水深无声 2021-11-02 15:24 382阅读 0赞

## 一、使用枚举类 ##

> 枚举

Python提供了`Enum`类来实现：

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。

可以对照着看一下输出结果：

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

### 自定义枚举类： ###

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)
    Weekday.Mon
    >>> print(Weekday.Tue)
    Weekday.Tue
    >>> print(Weekday['Tue'])
    Weekday.Tue
    >>> print(Weekday.Tue.value)
    2
    >>> print(day1 == Weekday.Mon)
    True
    >>> print(day1 == Weekday.Tue)
    False
    >>> print(Weekday(1))
    Weekday.Mon
    >>> print(day1 == Weekday(1))
    True
    >>> Weekday(7)
    Traceback (most recent call last):
      ...
    ValueError: 7 is not a valid Weekday
    >>> 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

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。

## 二、元类（metaclass） ##

> 当我们定义了类以后，就可以根据这个类创建出实例，所以：先定义类，然后创建实例。
> 
> 但是如果我们想创建出类呢？那就必须根据metaclass创建出类，所以：先定义metaclass，然后创建类。
> 
> 连接起来就是：先定义metaclass，就可以创建类，最后创建实例。
> 
> 所以，metaclass允许你创建类或者修改类。换句话说，你可以把类看成是metaclass创建出来的“实例”。

（待补）

其它博客链接：

1.  [变量和数据类型][Link 1]
2.  [list和tuple][list_tuple]
3.  [条件语句和循环][Link 2]
4.  [dict和set][dict_set]
5.  [函数][Link 3]
6.  [切片+迭代+生成器][Link 4]
7.  [生成器和迭代器][Link 5]
8.  [函数式编程][Link 6]
9.  [模块][Link 7]
10. [面向对象编程][Link 8]
11. [面向对象高级编程][Link 9]
12. [定制类][Link 10]
13. [枚举类和元类][Link 11]
14. [错误及处理][Link 12]

[Link 1]: https://blog.csdn.net/Vici__/article/details/99548213
[list_tuple]: https://blog.csdn.net/Vici__/article/details/99548479
[Link 2]: https://blog.csdn.net/Vici__/article/details/99548654
[dict_set]: https://blog.csdn.net/Vici__/article/details/99548727
[Link 3]: https://blog.csdn.net/Vici__/article/details/99548818
[Link 4]: https://blog.csdn.net/Vici__/article/details/99548933
[Link 5]: https://blog.csdn.net/Vici__/article/details/99549025
[Link 6]: https://blog.csdn.net/Vici__/article/details/99549108
[Link 7]: https://blog.csdn.net/Vici__/article/details/99549240
[Link 8]: https://blog.csdn.net/Vici__/article/details/99549279
[Link 9]: https://blog.csdn.net/Vici__/article/details/99549421
[Link 10]: https://blog.csdn.net/Vici__/article/details/99549548
[Link 11]: https://blog.csdn.net/Vici__/article/details/99549664
[Link 12]: https://blog.csdn.net/Vici__/article/details/99549808