code: def open_file(self, 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)
docs: Use local file or FTP depending on form of URL.