metaclass（元类）

一、创建类的执行流程

二、元类的认识

什么是元类呢？在Python3中继承type的就是元类

三、元类的示例

方式一：

  1 # 方式一
  2 class MyType(type):
  3     '''继承type的就是元类'''
  4     def __init__(self,*args,**kwargs):
  5         print("MyType创建的对象",self)   #Foo
  6         super(MyType,self).__init__(*args,**kwargs)
  7 
  8     def __call__(self, *args, **kwargs):
  9         obj = super(MyType,self).__call__(*args,**kwargs)
 10         print("类创建对象",self,obj)   #Foo
 11 
 12 class Foo(object,metaclass=MyType): #  对象加括号会去执行__call__方法，__call__方法里面继承了type的__call__方法
 13                                      ，type的__call__方法里面会先执行__new__方法，再去执行__init__方法。
 14                                       所以，Foo就是用type创建出来的
 15     user = "haiyan"
 16     age = 18
 17 
 18 obj = Foo()

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方式二

  1 # 方式二
  2 class MyType(type):
  3     def __init__(self, *args, **kwargs):
  4         print("ssss")
  5         super(MyType, self).__init__(*args, **kwargs)
  6 
  7     def __call__(cls, *args, **kwargs):
  8         v = dir(cls)
  9         obj = super(MyType, cls).__call__(*args, **kwargs)
 10         return obj
 11 #对象加括号就会去执行__call__方法
 12 class Foo(MyType('Zcc', (object,), {})):  #MyType('Zcc', (object,), {})相当于class Zcc(object):pass,也就是创建了一个Zcc的类
 13     user = 'haiyan'
 14     age = 18
 15 
 16 obj = Foo()

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方式三

  1 # 方式三
  2 class MyType(type):
  3     def __init__(self, *args, **kwargs):
  4         print("ssss")
  5         super(MyType, self).__init__(*args, **kwargs)
  6 
  7     def __call__(cls, *args, **kwargs):
  8         v = dir(cls)
  9         obj = super(MyType, cls).__call__(*args, **kwargs)
 10         return obj
 11 #对象加括号就会去执行__call__方法
 12 
 13 def with_metaclass(arg,base):
 14     print("类对象",MyType('Zcc', (base,), {}))
 15     return arg('Zcc', (base,), {})  #返回一个类对象  <class '__main__.Zcc'>
 16 
 17 class Foo(with_metaclass(MyType,object)):  #MyType('Zcc', (object,), {})相当于class Zcc(object):pass,也就是创建了一个Zcc的类
 18     user = 'haiyan'
 19     age = 18
 20 
 21 obj = Foo()

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=======================附加===========================

  1 class ASD(type):
  2     pass
  3 
  4 qqq = ASD("qwe", (object,), {})  #用ASD这个元类创建了一个（qwe,并且继承object类的）类
  5 
  6 # class ASD(qwe):
  7 #     pass
  8 obj = qqq()
  9 # 能创建类的是元类
 10 # 能创建对象的是类
 11 print(obj)  #<__main__.qwe object at 0x00000000024FFBA8>
 12 print(obj.__class__)  #<class '__main__.qwe'>
 13 print(obj.__class__.__class__)  #<class '__main__.ASD'>
 14 print(obj.__class__.__class__.__class__)  #<class 'type'>
 15 print(obj.__class__.__class__.__class__.__class__)  #<class 'type'>

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