code: def fetch(self, select=None, include=None):
          
          data = {}
          if select:
              if not isinstance(select, (list, tuple)):
                  raise TypeError('select parameter must be a list or a tuple')
              data['keys'] = ','.join(select)
          if include:
              if not isinstance(include, (list, tuple)):
                  raise TypeError('include parameter must be a list or a tuple')
              data['include'] = ','.join(include)
          response = client.get('/classes/{0}/{1}'.format(self._class_name, self.id), data)
          self._update_data(response.json())
docs: 从服务器获取当前对象所有的值，如果与本地值不同，将会覆盖本地的值。
  
          :return: 当前对象