第080讲：类和对象（XXIII）

小甲鱼

0. 本节视频




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Ensoleile

元类的应用

1. # 一、给所有的类添加一个属性:
   
2. class MetaC(type):
   
3.     def __new__(mcls, name, bases, attrs):
   
4.         attrs['author'] = 'fishc'
   
5.         return type.__new__(mcls, name, bases, attrs)
   
6. 
   
7. class C(metaclass=MetaC):
   
8.     pass
   
9. class D(metaclass=MetaC):
   
10.     pass
    
11. 
    
12. c = C()
    
13. d = D()
    
14. print(c.author, d.author)# fishc fishc
    
15. 
    
16. class MetaC(type):
    
17.     def __init__(cls, name, bases, attrs):
    
18.         cls.author = 'fishc'
    
19.         return type.__init__(cls, name, bases, attrs)
    
20. 
    
21. class C(metaclass=MetaC):
    
22.     pass
    
23. class D(metaclass=MetaC):
    
24.     pass
    
25. 
    
26. c = C()
    
27. d = D()
    
28. print(c.author, d.author)# fishc fishc
    
29. 
    
30. # 二、对类名的定义规范做限制
    
31. class MetaC(type):
    
32.     def __init__(cls, name, bases, attrs):
    
33.         if not name.istitle():
    
34.             raise TypeError('类名必须为大写字母开头！')
    
35.         type.__init__(cls, name, bases, attrs)
    
36. 
    
37. try:
    
38.     class mycls(metaclass=MetaC):
    
39.         pass
    
40. except TypeError as e:
    
41.     print(e)# 类名必须为大写字母开头！
    
42. 
    
43. # 三、修改对象的属性值 -> 如：把对象的所有字符串属性值都变成大写
    
44. class MetaC(type):
    
45.     def __call__(cls, *args, **kwargs):
    
46.         new_args = [each.upper() for each in args if isinstance(each, str)]
    
47.         return type.__call__(cls, *new_args, **kwargs)
    
48. 
    
49. class C(metaclass=MetaC):
    
50.     def __init__(self, name):
    
51.         self.name = name
    
52. 
    
53. c = C('FishC')
    
54. print(c.name)# FISHC
    
55. 
    
56. # 当然如果传入的不是str就不显示
    
57. class C(metaclass=MetaC):
    
58.     def __init__(self, *names):
    
59.         self.names = names
    
60. 
    
61. c = C('FishC', 'Python', 1223)
    
62. print(c.names)# ('FISHC', 'PYTHON')
    
63. 
    
64. # 四、限制类实例化时的传参方式 -> 如：要求类在实例化对象时只能关键字参数传参
    
65. class MetaC(type):
    
66.     def __call__(cls, *args, **kwargs):
    
67.         if args:
    
68.             raise TypeError('仅支持关键字参数~')
    
69.         return type.__call__(cls, *args, **kwargs)
    
70. 
    
71. class C(metaclass=MetaC):
    
72.     def __init__(self, name):
    
73.         self.name = name
    
74. 
    
75. try:
    
76.     c = C('fishc')
    
77. except TypeError as e:
    
78.     print(e)# 仅支持关键字参数~
    
79. 
    
80. c = C(name='fishc')
    
81. print(c.name)# fishc
    
82. 
    
83. # 五、禁止一个类被实例化
    
84. class NoInstances(type):
    
85.     def __call__(self, *args, **kwargs):
    
86.         raise TypeError('该类禁止实例化！')
    
87. 
    
88. class C(metaclass=NoInstances):
    
89.     pass
    
90. 
    
91. try:
    
92.     c = C()
    
93. except TypeError as e:
    
94.     print(e)# 该类禁止实例化！
    
95. 
    
96. # 禁止实例化对象也可以使用静态方法和类方法
    
97. class C(metaclass=NoInstances):
    
98.     @staticmethod
    
99.     def static_ok():
    
100.         print('静态方法允许使用~')
     
101.     @classmethod
     
102.     def class_ok(cls):
     
103.         print('类方法也允许使用~')
     
104. 
     
105. C.static_ok()# 静态方法允许使用~
     
106. C.class_ok()# 类方法也允许使用~
     
107. 
     
108. # 六、只允许实例化一个对象
     
109. class OnlyInstance(type):
     
110.     def __init__(cls, *args, **kwargs):
     
111.         cls.__instance = None
     
112.         type.__init__(cls, *args, **kwargs)
     
113.     def __call__(cls, *args, **kwargs):
     
114.         if cls.__instance is None:
     
115.             cls.__instance = type.__call__(cls, *args, **kwargs)
     
116.             return cls.__instance
     
117.         else:
     
118.             return cls.__instance
     
119. 
     
120. class C(metaclass=OnlyInstance):
     
121.     pass
     
122. 
     
123. c1 = C()
     
124. c2 = C()
     
125. print(c1 is c2)# True
     
126. # c1和c2为同一个对象
     
127. print(dir(C))
     
128. # ['_OnlyInstance__instance', '__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__']
     
129. # 对象被保存在类里边的_OnlyInstance__instance这个属性中
     

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