code: def render_string(self, template_name, **kwargs):
          
          if 'tornado' == settings['TEMPLATE_ENGINE']:
              return super(BaseHandler, self).render_string(template_name, **kwargs)
          elif 'jinja2' == settings['TEMPLATE_ENGINE']:
              return jinja2_render(template_name, **kwargs)
          else:
              raise errors.SettingsError(
                  '%s is not a supported TEMPLATE_ENGINE, should be `tornado` or `jinja2`'
                  % settings['TEMPLATE_ENGINE'])
docs: This method was rewritten to support multiple template engine
          (Determine by `TEMPLATE_ENGINE` setting, could be `tornado` and `jinja2`),
          it will only affect on template rendering process, ui modules feature,
          which is mostly exposed in `render` method, is kept to be used as normal.