设置带有复制警告的 pandas [英] Pandas SettingWithCopyWarning

问题描述

Python 3.4和Pandas 0.15.0

Python 3.4 and Pandas 0.15.0

df是一个数据框，而col1是一列.在下面的代码中，我正在检查值10的存在并将其替换为1000.

df is a dataframe and col1 is a column. With the code below, I'm checking for the presence of the value 10 and replacing such values with 1000.

df.col1[df.col1 == 10] = 1000

这是另一个例子.这次，我要根据索引更改col2中的值.

Here's another example. This time, I'm changing values in col2 based on index.

df.col2[df.index == 151] = 500

这两种情况都会产生以下警告:

Both these produce the warning below:

-c:1: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

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

最后，

cols = ['col1', 'col2', 'col3']
df[cols] = df[cols].applymap(some_function)

这会产生类似的警告，并带有建议:

This produces a similar warning, with an added suggestion:

Try using .loc[row_indexer,col_indexer] = value instead

我不确定我是否理解警告中指出的讨论.编写这三行代码的更好的方法是什么?

I'm not sure I understand the discussion pointed to in the warnings. What would be a better way to write these three lines of code?

请注意，该操作有效.

推荐答案

此处的问题是:df.col1[df.col1 == 10]返回一个副本.

The issue here is that: df.col1[df.col1 == 10] returns a copy.

所以我会说:

row_index = df.col1 == 10
# then with the form .loc[row_indexer,col_indexer]
df.loc[row_index, 'col1'] = 100

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