code: def _add_file(self, tar, name, contents, mode=DEFAULT_FILE_MODE):
          
          byte_contents = BytesIO(contents.encode('utf8'))
          info = tarfile.TarInfo(name=name)
          info.size = len(contents)
          # mtime must be 0 or any checksum operation
          # will return a different digest even when content is the same
          info.mtime = 0
          info.type = tarfile.REGTYPE
          info.mode = int(mode, 8)  # permissions converted to decimal notation
          tar.addfile(tarinfo=info, fileobj=byte_contents)
docs: Adds a single file in tarfile instance.
  
          :param tar: tarfile instance
          :param name: string representing filename or path
          :param contents: string representing file contents
          :param mode: string representing file mode, defaults to 644
          :returns: None