def create_plant_gen_id(df):
    """Combine the plant id and generator id to form a unique combination

    Parameters
    ----------
    df : dataframe
        Must contain columns plant_id_eia and generator_id

    Returns
    -------
    dataframe
        Same as input but with the additional column plant_gen_id
    """

    df["plant_gen_id"] = (
        df["plant_id_eia"].astype(str) + "_" + df["generator_id"].astype(str)
    )

    return df