使用元类 - python访问类中的属性

Question

我是python元类的新手并运行以下代码：

    class A(type):

        @classmethod
        def _handle_attrs(cls, class_name, bases, attrs, **kwargs):

            # Now lets find all handlers defined for this class
            current_attr = [(att, getattr(attrs[att], "__my_attr")) for att in attrs if
                          hasattr(attrs[att], '__my_attr')]

            print hasattr(attrs, '__my_attr') # returns False

        def __new__(cls, class_name, bases, attrs, **kwargs):
            cls._handle_attrs(class_name, bases, attrs, **kwargs)
            return type.__new__(cls, class_name, bases, attrs, **kwargs)


    # adding the decorators args as a class attribute

    def my_decorator(*args, **kwargs):
        def _my_decorator(cls):
            if hasattr(cls, '__my_attr'):
                cls.__my_attr.update(kwargs)
            else:
                cls.__my_attr = kwargs
            return cls
        return _my_decorator

    class B:
        __metaclass__ = A

        def get_attr_dict(self):
            return getattr(self, '__my_attr', None)


    @my_decorator(w = 'W')
    @my_decorator(z = 'Z')
    class C(B):
        x = 'X'
        y = 'Y'

    test = C()

    print test.__class__.__dict__ 
    print test.__my_attr # this returns {'w': 'W', 'z': 'Z'}

我通过使用类装饰器 - my_decorator将类属性添加到C类。在_handle_attrs函数中，我想对添加的属性做一些事情。

问题是添加的属性添加在：test._ _class __._ _dict__下，当在元类中创建C时，我不知道如何访问它们...

Answer 1

元类'__new__是在使用它的类的定义上运行的。以来

@my_decorator(w = 'W', z = 'Z')
class C(B):
    x = 'X'
    y = 'Y'

相当于

class C(B):
    x = 'X'
    y = 'Y'

C = my_decorator(w = 'W', z = 'Z')(C)

很明显，当装饰者添加其属性时，元类已经完成了它的工作。