Pandas：str.lower() 的 SettingWithCopyWarning

Question

I want to know how to deal with SettingWithCopyWarning when trying to apply str.lower() to a column in pandas dataframe so all my data, only on that column gets lowercase.

My actual code

df2["originator _ beliefs"]  = df2["originator _ beliefs"].str.lower()

My error:

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

My other attempts:

df3["originator _ beliefs"]  = df2.loc[df2["originator _ beliefs"].str.lower()]
df3["originator _ beliefs"]  = df2["originator _ beliefs"].str.lower().copy()

All I am trying is to lowercase all the items in "originator _ beliefs"

Answer 1

It's a warning, not an error. Check df2["originator _ beliefs"] after you run your code; you should see that it did indeed lower the column you were attempting to. The warning is referring to the fact that you are changing a copy of a slice from an inferred df1 (which is what df2 is) and that if you were trying to change df1 , you ain't gonna. There are ways to suppress the warning, but that is a bit out of scope of the question.

In [36]: df = pd.DataFrame([[1, 'a'], [2, 'b']], columns=['num', 'char'])

In [37]: df
Out[37]:
   num char
0    1    a
1    2    b

In [38]: df1 = df[df.char == 'a']

In [39]: df1
Out[39]:
   num char
0    1    a

In [40]: df1['char'] = df1['char'].str.upper()
<ipython-input-40-0ba8b77332da>:1: 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 caveats in the documentation: https://pandas.pydata.org/pandas-docs/stable/user_g
uide/indexing.html#returning-a-view-versus-a-copy
  df1['char'] = df1['char'].str.upper()

In [41]: df1
Out[41]:
   num char
0    1    A

In [42]: df
Out[42]:
   num char
0    1    a
1    2    b

Definitely check out the docs for more info on this: https://pandas.pydata.org/pandas-docs/stable/user_guide/indexing.html#returning-a-view-versus-a-copy