@classmethod
def _get_atr(cls, df, window=None):
    """ 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
        """
    if window is None:
        window = cls.ATR_SMMA
        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]
    cls._drop_columns(df, [tr_smma_column])
