descriptor.py

class descr_cls(object):

    def __init__(self):
        self.attr_in = 'was here in'

    def __setattr__(self, instance, value):
        print('set attr {}:{}'.format(instance, value))

        self.__dict__[instance] = value
        self.__dict__['{}_amended'.format(instance)] = (instance, value)

O = descr_cls()
O.attr1 = 'was here'

print('{}, {}'.format(O.attr1, O.attr_in))

print('{}, {}'.format(O.attr1_amended, O.attr_in_amended))