code: def users_sentiments_single_most_frequent(
          self,
          usernames = None,
          ):
          
          users_sentiments_single_most_frequent = dict()
          if usernames is None:
              usernames = self.usernames()
          try:
              for username in usernames:
                  sentiment = self.user_sentiments_most_frequent(
                      username             = username,
                      single_most_frequent = True
                  )
                  users_sentiments_single_most_frequent[username] = sentiment
              return users_sentiments_single_most_frequent
          except:
              log.error("error -- possibly a problem with tweets stored")
              return None
docs: This function returns the single most frequent calculated sentiment
          expressed by all stored users or by a list of specified users as a
          dictionary.