code: def format_string(self, s, args, kwargs):
          
          if isinstance(s, Markup):
              formatter = SandboxedEscapeFormatter(self, s.escape)
          else:
              formatter = SandboxedFormatter(self)
          kwargs = _MagicFormatMapping(args, kwargs)
          rv = formatter.vformat(s, args, kwargs)
          return type(s)(rv)
docs: If a format call is detected, then this is routed through this
          method so that our safety sandbox can be used for it.