基本常识
1.对象是类创建，创建对象时候类的__init__方法自动执行，对象()执行类的 __call__ 方法2.类是type创建，创建类时候type的__init__方法自动执行，类() 执行type的 __call__方法(类的__new__方法,类的__init__方法)# 第0步: 执行type的 __init__ 方法【类是type的对象】class Foo: ???def __init__(self): ???????pass ???def __call__(self, *args, **kwargs): ???????pass# 第1步: 执行type的 __call__ 方法# ???????1.1 ?调用 Foo类（是type的对象）的 __new__方法，用于创建对象。# ???????1.2 ?调用 Foo类（是type的对象）的 __init__方法，用于对对象初始化。obj = Foo()# 第2步：执行Foodef __call__ 方法obj()
示例一
class MyType(type): ???def __init__(self, *args, **kwargs): ???????print(‘MyType创建类‘,self) ???????super(MyType, self).__init__(*args, **kwargs) ???def __call__(self, *args, **kwargs): ???????obj = super(MyType, self).__call__(*args, **kwargs) ???????print(‘类创建对象‘, self, obj) ???????return objclass Foo(object,metaclass=MyType): ???user = ‘ctz‘ ???age = 18obj = Foo()
结果：
MyType创建类 <class ‘__main__.Foo‘>类创建对象 <class ‘__main__.Foo‘> <__main__.Foo object at 0x0000024858243470>
示例二:
class MyType(type): ???def __init__(self, *args, **kwargs): ???????print(‘MyType‘,self,‘----‘) ???????super(MyType, self).__init__(*args, **kwargs) ???def __call__(cls, *args, **kwargs): ???????v = dir(cls) ???????obj = super(MyType, cls).__call__(*args, **kwargs) ???????print(‘MyType‘,cls,obj,‘****‘) ???????return objclass Foo(MyType(‘MyType‘, (object,), {})): ???user = ‘ctz‘ ???age = 18obj = Foo()结果
MyType <class ‘__main__.MyType‘> ----MyType <class ‘__main__.Foo‘> ----MyType <class ‘__main__.Foo‘> <__main__.Foo object at 0x0000025A92BB36D8> ****
示例三:(查看wtforms源码可知,实例化Form就是通过这种方法做的,此外这种方式可以实现单例模式)
class MyType(type): ???def __init__(self, *args, **kwargs): ???????print(self,‘------‘) ???????super(MyType, self).__init__(*args, **kwargs) ???def __call__(cls, *args, **kwargs): ???????v = dir(cls) ???????obj = super(MyType, cls).__call__(*args, **kwargs) ???????print(cls,obj,‘****‘) ???????return objdef with_metaclass(arg,base): ???return MyType(‘MyType‘, (base,), {})class Foo(with_metaclass(MyType,object)): ???user = ‘ctz‘ ???age = 18obj = Foo()结果:
<class ‘__main__.MyType‘> ------<class ‘__main__.Foo‘> ------<class ‘__main__.Foo‘> <__main__.Foo object at 0x000001FBD9E63710> ****
metaclass
原文地址：https://www.cnblogs.com/lianxuebin/p/8664268.html