Python，lambda 函数作为 groupby 的参数

Question

我试图弄清楚一段代码在做什么，但我有点迷路了。

我有一个 pandas 数据框，它已由以下 .csv 文件加载:

origin_census_block_group,date_range_start,date_range_end,device_count,distance_traveled_from_home,bucketed_distance_traveled,median_dwell_at_bucketed_distance_traveled,completely_home_device_count,median_home_dwell_time,bucketed_home_dwell_time,at_home_by_each_hour,part_time_work_behavior_devices,full_time_work_behavior_devices,destination_cbgs,delivery_behavior_devices,median_non_home_dwell_time,candidate_device_count,bucketed_away_from_home_time,median_percentage_time_home,bucketed_percentage_time_home,mean_home_dwell_time,mean_non_home_dwell_time,mean_distance_traveled_from_home
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df = pd.read_csv(csv_file,
        usecols=[
                'origin_census_block_group',
                'date_range_start',
                'date_range_end',
                'device_count',
                'distance_traveled_from_home',
                'completely_home_device_count',
                'median_home_dwell_time',
                'part_time_work_behavior_devices',
                'full_time_work_behavior_devices'
                ],
                    dtype={'origin_census_block_group': str},
                ).set_index('origin_census_block_group')

并且，稍后在代码中，数据框被修改为:

df = df.groupby(lambda cbg: cbg[:5]).sum()

我不太明白这条线到底在做什么。Groupby 通常按列对数据框进行分组，所以......它是否使用多列(0 到 5)对数据框进行分组？ .sum() 最后的作用是什么？

Answer 1

如果您完全按照编写的方式运行代码(创建 df 和 groupby)，您可以看到结果。我打印 groupby

输出的前几列

         device_count    distance_traveled_from_home
-----  --------------  -----------------------------
01053              49                            626
01073             139                           2211
01089              78                           1934
01095             100                           2481

这里发生的是函数 lambda cbg: cbg[:5] 应用于每个索引值(看起来像列 origin_census_block_group 中的数字的字符串)。作为一面，请注意声明

...
dtype={'origin_census_block_group': str},

在创建 df 时，有人遇到麻烦以确保它们实际上是 str

因此该函数应用于 '010539707003' 之类的字符串，并返回一个子字符串，即该字符串的前 5 个字符:

'010539707003'[:5]

产生

'01053'

所以我假设有多个键共享前 5 个字符(在实际文件中——代码段中它们都是唯一的，所以不是很有趣)并且所有这些行都组合在一起

然后 .sum() 应用于每个组的每个数字列并返回每个 groupby 键的列总和。这就是您在我的 'device_count' 列等输出中看到的内容。

希望现在一切都清楚了