class Field(object):
    def __get__(self, instance, owner):
        print('__get__', self, instance, self.name)
        return getattr(instance, self.name)
    def __set__(self, instance, value):
        print('__set__', self, instance, self.name, value)
        setattr(instance, self.name, value)

class Meta(type):
    def __new__(meta, name, bases, class_dict):
        print('__new__', name, bases, class_dict)
        for name, value in class_dict.items():
            if isinstance(value, Field):
                setattr(value, 'name', '_' + name)
        return type.__new__(meta, name, bases, class_dict)

class Model(object, metaclass=Meta):
    pass

class Record(Model):
    a = Field()
    b = Field()

r = Record()
r.a = 'Bob'
r.b = 1234