在DataFrame的切片副本上设置值 [英] Setting values on a copy of a slice from a DataFrame

问题描述

我有一个小的数据框，说一个:

I have a small dataframe, say this one :

    Mass32      Mass44  
12  0.576703    0.496159
13  0.576658    0.495832
14  0.576703    0.495398    
15  0.576587    0.494786
16  0.576616    0.494473
...

我希望列Mass32的滚动平均值，所以我这样做:

I would like to have a rolling mean of column Mass32, so I do this:

x['Mass32s'] = pandas.rolling_mean(x.Mass32, 5).shift(-2)

它的工作原理与我有一个名为Mass32s的新列一样，其中包含了我希望包含的内容，但我也收到警告消息:

It works as in I have a new column named Mass32s which contains what I expect it to contain but I also get the warning message:

试图在DataFrame的切片副本上设置一个值.尝试 改用.loc [row_indexer，col_indexer] =值

A value is trying to be set on a copy of a slice from a DataFrame. Try using .loc[row_indexer,col_indexer] = value instead

请参阅文档中的注意事项: http://pandas.pydata.org/pandas -docs/stable/indexing.html#indexing-view-versus-copy

See the the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy

我想知道是否有更好的方法，特别是避免收到此警告消息.

I'm wondering if there's a better way to do it, notably to avoid getting this warning message.

推荐答案

出现此警告是因为数据框x是切片的副本.这不容易知道为什么，但是与您如何达到当前状态有关.

This warning comes because your dataframe x is a copy of a slice. This is not easy to know why, but it has something to do with how you have come to the current state of it.

您可以通过x在x中创建合适的dataframe

You can either create a proper dataframe out of x by doing

x = x.copy()

这将删除警告，但这是不正确的方式

This will remove the warning, but it is not the proper way

如警告所示，您应该使用DataFrame.loc方法，如下所示:

You should be using the DataFrame.loc method, as the warning suggests, like this:

x.loc[:,'Mass32s'] = pandas.rolling_mean(x.Mass32, 5).shift(-2)

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