python全栈开发基础【补充】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

二、元类的示例

# 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 = "haiyan"
    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()

附加

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'>