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

class MelonCart:
    honeymelons = Positive('honeymelons')
    watermelons = Positive('watermelons')

    def __init__(self, honeymelons, watermelons):
        self.honeymelons = honeymelons
        self.watermelons = watermelons