@classmethod
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] = []
