使用loc时的Pandas SettingWithCopyWarning [英] Pandas SettingWithCopyWarning When Using loc

问题描述

使用.loc对索引/切片的赋值进行一般性提问。

Have a general question on assignments with indexing/slicing using .loc.

假设下面的DataFrame，df：

Assume the below DataFrame, df:

df:    
    A   B   C
0   a   b   
1   a   b   
2   b   a   
3   c   c   
4   c   a   

要重现的代码：

df = pd.DataFrame({'A':list('aabcc'), 'B':list('bbaca'), 'C':5*[None]})

我使用以下方式创建df1：

I create df1 using:

df1=df.loc[df.A=='c']

df1:
    A   B   C
3   c   c   
4   c   a   

然后我根据B中的值使用以下内容为C赋值：

I then assign a value to C based upon a value in B using:

df1.loc[df1.B=='a','C']='d'

作业工作，但我收到一个SettingWithCopy警告。我做错了什么或者这是预期的功能吗？我认为使用.loc会避免链接赋值。有什么东西我不见了吗？我正在使用Pandas 14.1

The assignment works, but I receive a SettingWithCopy warning. Am I doing something wrong or is this the expected functionality? I thought that using .loc would avoid chained assignment. Is there something that I am missing? I am using Pandas 14.1

推荐答案

@EdChum在评论中回答OP已经解决了这个问题。
即替换

@EdChum answer in comments to OP has solved the issue. i.e. replace

df1=df.loc[df.A=='c']

with

df1=df.loc[df.A=='c'].copy()

这样可以明确表达您的意图而不会发出警告

this will make it clear your intentions and not raise a warning

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