code: def quality(self, key):
          
          for item, quality in self:
              if self._value_matches(key, item):
                  return quality
          return 0
docs: Returns the quality of the key.
  
          .. versionadded:: 0.6
             In previous versions you had to use the item-lookup syntax
             (eg: ``obj[key]`` instead of ``obj.quality(key)``)