Python descriptor

class A:

    def __init__(self, name):

        self.name = name

    def __get__(self, ins, cls):

        print('call get')

        if ins is None:

            return self

        return ins.__dict__[self.name]

    def __set__(self, ins, val):

        print('call set')

        ins.__dict__[self.name] = val

    def __delete__(self, ins):

        print('cal del')

        del ins.__dict__[self.name]

class B:

    a1 = A('a1')

    def __init__(self):

        self.a1 = 3

#b=B.__new__(B)

b=B()

b.a1

#B.a1