old_contents: class Stock:
      def __init__(self, symbol):
          self.symbol = symbol
          self.price = None
  
      def update(self, timestamp, price):
          self.price = price
  
new_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
  
subject: Update update method for negative price exception.