old_contents: 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
  
new_contents: 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)
  
subject: Update price attribute to price_history list as well as update function accordingly.