docs: def sendline(self, cmd):
          '''
          Send this command to the server and
          return a tuple of the output and the stderr.
  
          The format for parameters is:
  
          cmd (string): The command to send to the sever.
          '''
          self.conn.sendline(cmd, self.linesep)
  
          # saw_prompt = False
          ret_stdout = []
          ret_stderr = []
          while self.conn.has_unread_data:
              stdout, stderr = self.conn.recv()
  
              if stdout:
                  ret_stdout.append(stdout)
              if stderr:
                  log.debug('Error while executing command.')
                  ret_stderr.append(stderr)
  
              if stdout and self.prompt_re.search(stdout):
                  break
  
          return ''.join(ret_stdout), ''.join(ret_stderr)