code: def contians_attribute(self, attribute):
          
          if self.library is None:
              return 0
  
          load = self.library.load_card
          matches = 0
          for code in self.cards:
              card = load(code)
              if card.has_attribute(attribute):
                  matches += 1
          return matches
docs: Returns how many cards in the deck have the specified attribute.
  
          This method requires a library to be stored in the deck instance and
          will return `None` if there is no library.