descriptor_demo.py

class NonNegativeDescriptor(object):
    def __init__(self, name):
        self.name = name

    def __get__(self, obj, type_):
        return obj.data.get(self.name, 0)

    def __set__(self, obj, value):
        if value < 0:
            value = 0
        obj.data[self.name] = value


class Player(object):
    def __init__(self):
        self.data = {}

    hp = NonNegativeDescriptor('hp')
    mp = NonNegativeDescriptor('mp')


if __name__ == '__main__':
    p = Player()

    p.hp = 10
    p.mp = -1

    print (p.hp, p.mp)  # (10, 0)