@classmethod
def _get_smma(cls, df, column, windows):
    """ get smoothed moving average.

        :param df: data
        :param windows: range
        :return: result series
        """
    window = cls.get_only_one_positive_int(windows)
    column_name = '{}_{}_smma'.format(column, window)
    smma = df[column].ewm(ignore_na=False, alpha=1.0 / window, min_periods=
        0, adjust=True).mean()
    df[column_name] = smma
    return smma
