code: def sortTitles(self, by):
          titles = sorted(iter(self))
          if by == 'length':
              return sorted(
                  titles, reverse=True,
                  key=lambda title: self[title].subjectLength)
          if by == 'maxScore':
              return sorted(
                  titles, reverse=True, key=lambda title: self[title].bestHsp())
          if by == 'medianScore':
              return sorted(
                  titles, reverse=True,
                  key=lambda title: self.scoreClass(self[title].medianScore()))
          if by == 'readCount':
              return sorted(
                  titles, reverse=True,
                  key=lambda title: self[title].readCount())
          if by == 'title':
              return titles
          raise ValueError('Sort attribute must be one of "length", "maxScore", '
                           '"medianScore", "readCount", "title".')