code: def Open(self):
      
      if self._is_open:
        raise IOError('Storage writer already opened.')
  
      self._is_open = True
  
      self._first_written_event_source_index = len(self._event_sources)
      self._written_event_source_index = self._first_written_event_source_index
docs: Opens the storage writer.
  
      Raises:
        IOError: if the storage writer is already opened.
        OSError: if the storage writer is already opened.