def rememberRootURL(self):
        """
        Remember the currently-processed part of the URL for later
        recalling.
        """
        url = self._prePathURL(self.prepath[:-1])
        self.appRootURL = url

        Remember the currently-processed part of the URL for later
        recalling.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/web/server.py