docs: def render_string(self, source: str, **vars) -> str:
          """
          Render the template contained in the given string.
  
          The current context will be available to the template as the ``ctx`` variable.
  
          :param source: content of the template to render
          :param vars: extra variables made available to the template
          :return: the rendered results
  
          """
          vars.setdefault('ctx', self._ctx)
          return self._renderer.render_string(source, **vars)