Pandas map to a new column, SettingWithCopyWarning

Question

In pandas data frame, I'm trying to map df['old_column'], apply user defined function f for each row and create a new column.

df['new_column'] = df['old_column'].map(lambda x: f(x))

This will give out "SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame." error.

I tried the following:

df.loc[:, 'new_column'] = df['old_column'].map(lambda x: f(x))

which doesn't help. What can I do?

In pandas 0.19.2 this warning appears no longer to be raised in this circumstance — Ninjakannon, May 13 '17 at 15:17
The warning actually explicitly says to try using `.loc` instead, but when following that advice the warning is still raised. Quite confusing. (version 1.2.0) — rocksNwaves, Mar 11 '21 at 00:05

score 19 · Answer 1 · edited May 23 '17 at 11:54

19

A SettingWithCopy warning is raised for certain operations in pandas which may not have the expected result because they may be acting on copies rather than the original datasets. Unfortunately there is no easy way for pandas itself to tell whether or not a particular call will or won't do this, so this warning tends to be raised in many, many cases where (from my perspective as a user) nothing is actually amiss.

Both of your method calls are fine. If you want to get rid of the warning entirely, you can specify:

pd.options.mode.chained_assignment = None

See this StackOverflow Q&A for more information on this.

edited May 23 '17 at 11:54

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answered Feb 08 '17 at 06:20

Aleksey Bilogur

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I suggest putting a `pd.options.mode.chained_assignment = 'warn'` after the triggering statement, to catch possible true positives of this warning in your subsequent code. – Philipp Feb 12 '21 at 14:36

Pandas map to a new column, SettingWithCopyWarning

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