code: def _load_prev(self):
          
  
          if self._load_by_date:
              prev_date = self.date - pds.DateOffset(days=1)
              return self._load_data(date=prev_date)
          else:
              return self._load_data(fid=self._fid-1)
docs: Load the next days data (or file) without decrementing the date.
          Repeated calls will not decrement date/file and will produce the same
          data
          
          Uses info stored in object to either decrement the date, 
          or the file. Looks for self._load_by_date flag.