code: def _extract(self, source, *args, **kwargs):
          
          # Extract data
          self._data = mbox_to_pandas(source)
          self._data['MessageID'] = pd.Series(range(0,len(self._data)))
docs: Extracts data from mbox files. Mutates _data.
  
          :param str source: The path to one or more mbox files.
          :param args: Arbitrary arguments for extensibility.
          :param kwargs: Arbitrary keyword arguments for extensibility.
          :return: None