def excess_pore_pressure_ratio(df):
    """
    Assign the excess pore pressure ratio, if the water pressure u2 is defined. Else, raise ERROR.

    :param df: (DataFrame)
    :return: (DataFrame)
    """
    try:
        u2 = df['u2']
    except KeyError:
        raise SystemExit(
            'ERROR: u2 not defined in .gef file, change classifier')
    return df.assign(excess_pore_pressure_ratio=(u2 - df['water_pressure']) /
        (df['qt'] - df['soil_pressure']))
