class Stock:
    def __init__(self, symbol):
        self.symbol = symbol
        self.price = None

    def update(self, timestamp, price):
        if price < 0:
            raise ValueError("price should not be negative")
        self.price = price

class Stock:
    def __init__(self, symbol):
        self.symbol = symbol
        self.price_history = []

    @property
    def price(self):
        if self.price_history:
            return self.price_history[-1]
        else:
            return None

    def update(self, timestamp, price):
        if price < 0:
            raise ValueError("price should not be negative")
        self.price_history.append(price)

Update price attribute to price_history list as well as update function accordingly.
Update price attribute to price_history list as well as update function accordingly.
