Python获取SettingWithCopyWarning-iloc与loc-无法找出原因

Question

我对SettingWithCopyWarning有基本的了解，但是我无法弄清楚为什么我收到此特定情况的警告。

我正在遵循https://github.com/ageron/handson-ml/blob/master/02_end_to_end_machine_learning_project.ipynb中的代码

当我运行以下代码（使用.loc）时，没有得到SettingWithCopyWarning

但是，如果我改为使用.iloc运行代码，则会收到警告。

有人可以帮我理解吗？

from sklearn.model_selection import StratifiedShuffleSplit

split = StratifiedShuffleSplit(n_splits=1, test_size=0.2, random_state=42)

for train_index, test_index in split.split(housing, housing["income_cat"]):
    strat_train_set = housing.loc[train_index]
    strat_test_set = housing.loc[test_index]

for set_ in (strat_train_set, strat_test_set):
    set_.drop("income_cat", axis=1, inplace=True)

Answer 1

The issue here is not because of indexing, iloc and loc would work the same way for you here. The problem is in set_.drop("income_cat", axis=1, inplace=True). It looks like there's a weak reference between the set_ data frame and the strat_train_set and strat_test_set.

for set_ in (strat_train_set, strat_test_set):
         print(set_._is_copy)

With this you get:

<weakref at 0x128b30598; to 'DataFrame' at 0x128b355c0>
<weakref at 0x128b30598; to 'DataFrame' at 0x128b355c0>

This could lead to SettingWithCopyWarning as it's trying to transform the copy of the data frame and applying those change to the original ones as well.