code: def _get_atr(cls, df, window=None):
          
          if window is None:
              window = 14
              column_name = 'atr'
          else:
              window = int(window)
              column_name = 'atr_{}'.format(window)
          tr_smma_column = 'tr_{}_smma'.format(window)
  
          df[column_name] = df[tr_smma_column]
          del df[tr_smma_column]
docs: Average True Range
  
          The average true range is an N-day smoothed moving average (SMMA) of
          the true range values.  Default to 14 days.
          https://en.wikipedia.org/wiki/Average_true_range
  
          :param df: data
          :return: None