new_contents: import unittest
  
  
  class Stock:
      def __init__(self, symbol):
          self.symbol = symbol
          self.price = None
  
  
  class StockTest(unittest.TestCase):
      def test_price_of_a_new_stock_class_should_be_None(self):
          stock = Stock("GOOG")
          self.assertIsNone(stock.price)
  
  if __name__ == "__main__":
      unittest.main()
  
subject: Add stock class.