Why I get "A value is trying to be set on a copy of a slice from a DataFrame"

Asked May 05 '19 at 07:51

Active May 05 '19 at 08:06

Viewed 84 times

I have a data frame called df, with column A. I do the following code:

df['A'][df['A'] < 4] = np.NaN

I get the following message :

A value is trying to be set on a copy of a slice from a DataFrame

See the caveats in the documentation: http://pandas.pydata.org/pandas-docs/stable/indexing.html#indexing-view-versus-copy
df['A'][df['A'] < 4] = np.NaN
bla.py:763: SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame

The code works, df is changed after executing this line, so In practice it does not trying to change a copy of df but the original df

Why do I get this warning ?

P.S : I looked at similar questions on stack overflow but could't find an answer

edited May 05 '19 at 08:06

asked May 05 '19 at 07:51

OopsUser

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Use `.loc` to assign value `df.loc[df['A'] < 4, 'A'] = np.nan` – ResidentSleeper May 05 '19 at 07:58
Similar question: https://stackoverflow.com/questions/20625582/how-to-deal-with-settingwithcopywarning-in-pandas – Anshul Sharma May 05 '19 at 08:06
@ResidentSleeper maybe the `.loc` will remove the warning, but still it doesn't explain why i'm getting a warning in the first place. – OopsUser May 05 '19 at 13:45
@OopsUser pretty good explanation in the answer linked by Anshul. – Sid May 05 '19 at 19:14

Why I get "A value is trying to be set on a copy of a slice from a DataFrame"

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