code: def _patchheader(self):
      if self._form_length_pos is None:
          raise OSError("cannot seek")
      self._file.seek(self._form_length_pos)
      _write_u32(self._file, self._datawritten)
      self._datalength = self._datawritten
      self._file.seek(0, 2)
  
imports: from collections import namedtuple
  import warnings
label_desc: Use image node_id 25 for calling the Au_write obj's underlying member method code with example usage: obj._patchheader() without return types