def add_gid(df):
    """ Add a column to game df with unique game id """

    def get_gid(row):
        tid1, tid2 = sorted((row.WTeamID, row.LTeamID))
        return '{0}_{1}_{2}'.format(row.Date, tid1, tid2)

    df['gid'] = df.apply(get_gid, axis=1)