class _Positive:
    def __init__(self, attr):
        self.attr = attr # the managed attribute
        self.msg = "can't have less than zero of {}!".format(attr)

    def __set__(self, instance, value):
        if value < 0:
            raise ValueError(self.msg)
        vars(instance)[self.attr] = value

def positive(*attrs):
    def set_attrs(cls):
        for attr in attrs:
            setattr(cls, attr, _Positive(attr))
        return cls
    return set_attrs

@positive('honeymelons', 'watermelons')
class MelonCart:
    def __init__(self, honeymelons, watermelons):
        self.honeymelons = honeymelons
        self.watermelons = watermelons