If a format call is detected, then this is routed through this
        method so that our safety sandbox can be used for it.

def format_string(self, s, args, kwargs):
        """If a format call is detected, then this is routed through this
        method so that our safety sandbox can be used for it.
        """
        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)