code: def _get_index(self, server_url):
          
          for i in range(len(self.sessions)):
              session = self.sessions[i]
              if session.url == server_url:
                  return i
          return -1
docs: Returns a registered GPServer object with a matching GenePattern server url
          Returns -1 if no matching result was found
          :param server_url:
          :return: