def open_file(self, url):
        """Use local file or FTP depending on form of URL."""
        if not isinstance(url, str):
            raise URLError('file error: proxy support for file protocol currently not implemented')
        if url[:2] == '//' and url[2:3] != '/' and url[2:12].lower() != 'localhost/':
            raise ValueError("file:// scheme is supported only on localhost")
        else:
            return self.open_local_file(url)
Use local file or FTP depending on form of URL.