如何避免“试图在DataFrame的切片副本上设置值"? [英] How to avoid 'A value is trying to be set on a copy of a slice from a DataFrame'?

问题描述

我有一个数据框，df_original:

     a     b
0    10    5
1    12    6
2    14    1

现在，我想创建一个包含所有行在c > 5的新数据框，然后在此新数据框上设置新的列值:

Now I want to make a new dataframe containing all rows where c > 5, and then set a new column value on this new dataframe:

df = df_original[df_original['b'] > 5]
df['c'] = df['a'] / df['b']

但是我得到警告:

 /Library/Python/2.7/site-packages/ipykernel/__main__.py:25: SettingWithCopyWarning: 
 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
 See the the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy

如果c不返回原始数据框，我会感到非常高兴.但是我应该如何正确执行操作并避免出现警告?

I'm quite happy if c doesn't make its way back to the original dataframe. But how should I do this properly and avoid warnings?

推荐答案

采取

Take a copy to avoid the warning:

In [118]:
df1 = df[df['b'] > 5].copy()
df1['c'] = df1['a'] / df1['b']
df1

Out[118]:
    a  b    c
1  12  6  2.0

这避免了通话中的歧义

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