metaclass元类解析

Posted 2020-10-21 Sober--

一、创建类的流程

 

 二、什么是元类

在Python3中继承type的就是元类

示例

# 方式一
class MyType(type):
    \'\'\'继承type的就是元类\'\'\'
    def __init__(self,*args,**kwargs):
        print("MyType创建的对象",self)   #Foo
        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)   #Foo

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
                                     ，type的__call__方法里面会先执行__new__方法，再去执行__init__方法。
                                      所以，Foo就是用type创建出来的
    user = "haha"
    age = 18

obj = Foo()

# 方式二
class MyType(type):
    def __init__(self, *args, **kwargs):
        print("ssss")
        super(MyType, self).__init__(*args, **kwargs)

    def __call__(cls, *args, **kwargs):
        v = dir(cls)
        obj = super(MyType, cls).__call__(*args, **kwargs)
        return obj
#对象加括号就会去执行__call__方法
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
    user = \'haiyan\'
    age = 18

obj = Foo()

# 方式三
class MyType(type):
    def __init__(self, *args, **kwargs):
        print("ssss")
        super(MyType, self).__init__(*args, **kwargs)

    def __call__(cls, *args, **kwargs):
        v = dir(cls)
        obj = super(MyType, cls).__call__(*args, **kwargs)
        return obj
#对象加括号就会去执行__call__方法

def with_metaclass(arg,base):
    print("类对象",MyType(\'Zcc\', (base,), {}))
    return arg(\'Zcc\', (base,), {})  #返回一个类对象  <class \'__main__.Zcc\'>

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
    user = \'haiyan\'
    age = 18

obj = Foo()

class ASD(type):
    pass

qqq = ASD("qwe", (object,), {})  #用ASD这个元类创建了一个（qwe,并且继承object类的）类

# class ASD(qwe):
#     pass
obj = qqq()
# 能创建类的是元类
# 能创建对象的是类
print(obj)  #<__main__.qwe object at 0x00000000024FFBA8>
print(obj.__class__)  #<class \'__main__.qwe\'>
print(obj.__class__.__class__)  #<class \'__main__.ASD\'>
print(obj.__class__.__class__.__class__)  #<class \'type\'>
print(obj.__class__.__class__.__class__.__class__)  #<class \'type\'>

其它