metaclass

Posted 2020-10-28 East-L

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 obj


class Foo(object,metaclass=MyType):
    user = ‘ctz‘
    age = 18

obj = Foo()

View Code

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 obj


class Foo(MyType(‘MyType‘, (object,), {})):
    user = ‘ctz‘
    age = 18


obj = Foo()

View Code

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 obj


def with_metaclass(arg,base):
    return MyType(‘MyType‘, (base,), {})


class Foo(with_metaclass(MyType,object)):
    user = ‘ctz‘
    age = 18


obj = Foo()

View Code

结果:

<class ‘__main__.MyType‘> ------
<class ‘__main__.Foo‘> ------
<class ‘__main__.Foo‘> <__main__.Foo object at 0x000001FBD9E63710> ****