如何通过.loc读取熊猫中的特定行？

Question

我想计算每小时的平均流量并将其保存在新的数据框中，以最终导出到excel。 当我定义一个满足我强加条件的新数据框时，它将返回仅包含标头的新数据框。以下是我对df2有问题的脚本的一部分：

for year in range(2018,2020):
    for month in range(1,13):
        for day in range(1,32):
            for hour in range(0,24):
                df2= df.loc[(df['YYYY'] == year) & (df['MM'] == month) & (df['DD'] == day) & (df['hh'] == hour)]                
                res_frame.at[i, 'year'] = year
                res_frame.at[i, 'month'] = month
                res_frame.at[i, 'day'] = day
                res_frame.at[i, 'hour'] = hour
                res_frame.at[i, 'average_flow'] = df2.iloc[:,7].mean(axis=0)  
                i +=1