实现CarForSale零向量

class CarForSale(Vector):
    retrieved_date = datetime(2018,11,30,12) #1
    def __init__(self, model_year, mileage, price, posted_datetime, 
                 model="(virtual)", source="(virtual)", #2
                 location="(virtual)", description="(virtual)"):
        self.model_year = model_year
        self.mileage = mileage
        self.price = price
        self.posted_datetime = posted_datetime
        self.model = model
        self.source = source
        self.location = location
        self.description = description
    def add(self, other):
        def add_dates(d1, d2): #3
            age1 = CarForSale.retrieved_date - d1
            age2 = CarForSale.retrieved_date - d2
            sum_age = age1 + age2
            return CarForSale.retrieved_date - sum_age
        return CarForSale( #4
            self.model_year + other.model_year,
            self.mileage + other.mileage,
            self.price + other.price,
            add_dates(self.posted_datetime, other.posted_datetime)
        )
    def scale(self,scalar):
        def scale_date(d): #5
            age = CarForSale.retrieved_date - d
            return CarForSale.retrieved_date - (scalar * age)
        return CarForSale(
            scalar * self.model_year,
            scalar * self.mileage,
            scalar * self.price,
            scale_date(self.posted_datetime)
        )
    @classmethod
    def zero(cls):
        return CarForSale(0, 0, 0, CarForSale.retrieved_date)