def _get_ema(cls, df, column, windows):
        """ get exponential moving average

        :param df: data
        :param column: column to calculate
        :param windows: collection of window of exponential moving average
        :return: None
        """
        window = cls.get_only_one_positive_int(windows)
        column_name = '{}_{}_ema'.format(column, window)
        if len(df[column]) > 0:
            df[column_name] = df[column].ewm(
                ignore_na=False, span=window,
                min_periods=0, adjust=True).mean()
        else:
            df[column_name] = []