Pandas 数据框：set_index with inplace=True 返回 NoneType，为什么？
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Pandas dataframe: set_index with inplace=True returns a NoneType, why?
提问by Wouter
If I reset the index of my Pandas dataframe with "inplace=True" (following the documentation) it returns a class 'NoneType'. If I reset the index with "inplace=False" it returns the dataframe with the new index. Why?
如果我使用“inplace=True”（按照文档）重置我的 Pandas 数据框的索引，它会返回一个类“NoneType”。如果我用“inplace=False”重置索引，它会返回带有新索引的数据帧。为什么？
print(type(testDataframe))
print(testDataframe.head())
returns:
返回：
<class 'pandas.core.frame.DataFrame'>
ALandbouwBosbouwEnVisserij AantalInkomensontvangers AantalInwoners \
0 73780.0 None 16979120
1 290.0 None 25243
2 20.0 None 3555
Set_index returns a new index:
Set_index 返回一个新索引：
testDataframe = testDataframe.set_index(['Codering'])
print(type(testDataframe))
print(testDataframe.head())
returns
回报
<class 'pandas.core.frame.DataFrame'>
ALandbouwBosbouwEnVisserij AantalInkomensontvangers \
Codering
NL00 73780.0 None
GM1680 290.0 None
WK168000 20.0 None
BU16800000 15.0 None
But the same set_index with "inplace=True":
但同样的 set_index 与“inplace=True”：
testDataframe = testDataframe.set_index(['Codering'], inplace=True)
print(type(testDataframe))
print(testDataframe.head())
returns
回报
<class 'NoneType'>
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-50-0d6304ebaae1> in <module>()
Version info:
版本信息：
python: 3.4.4.final.0
python-bits: 64
pandas: 0.18.1
numpy: 1.11.1
IPython: 5.2.2
回答by Wouter
Ok, now I understand, thanks for the comments!
好的，现在我明白了，感谢您的评论！
So inplace=True should return None andmake the change in the original object. It seemed that on listing the dataframe again, no changes were present.
所以 inplace=True 应该返回 None并在原始对象中进行更改。似乎在再次列出数据框时，没有任何变化。
But of course I should not have assigned the return valueto the dataframe, i.e.
但是当然我不应该将返回值分配给数据框，即
testDataframe = testDataframe.set_index(['Codering'], inplace=True)
should just be
应该只是
testDataframe.set_index(['Codering'], inplace=True)
or
或者
testDataframe = testDataframe.set_index(['Codering'], inplace=False)
otherwise the return value of the inplace index change (None) is the new content of the dataframe which is of course not the intend.
否则就地索引更改（无）的返回值是数据帧的新内容，这当然不是预期的。
I am sure this is obvious to many and now it is to me as well but it wasn't without your help, thanks!!!
我相信这对很多人来说都是显而易见的，现在对我来说也是如此，但这并非没有你的帮助，谢谢！！！
回答by Shubham gupta
inplace=True is always changed in the original data_frame. If you want a changed data_frame then remove second parameter i.e inplace = True
inplace=True 始终在原始数据帧中更改。如果你想要一个改变的 data_frame 然后删除第二个参数即inplace = True
new_data_frame = testDataframe.set_index(['Codering'])
Then
然后
print(new_data_frame)
