Python Pandas SettingWithCopyWarning while creating new column

Question

I have a dataframe df as:

df.iloc[1:5,1:3]
                   Date  Month
4   2013-01-03 00:00:00      1
6   2013-01-04 00:00:00      1
10  2013-01-07 00:00:00      1
12  2013-01-08 00:00:00      1

I am trying the following:

df['newCol'] = df['Month']*2

I get the following warning:

<input>: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

What is the correct way to do the above?

Arn · Accepted Answer · 2020-03-08 13:51:17Z

2

In this case, it is safe to assign the value the way you did. However, if you want to avoid the warning to keep the good habit, you can do what the message says, i.e.:

df.loc[:, 'newCol'] = df['Month']*2

answered Mar 8, 2020 at 13:51

Arn

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2 Comments

Arn Over a year ago

@gyaan.anveyshak I assume you are referring to a different question. The issue is not with the method I suggested but with the preliminary steps, in which you are just creating an alias of a dataframe instead of copying it as indicated in the answer to your question.

rahul-ahuja Over a year ago

Yes, the problem was with one of the preliminary steps. I figured it out only after I posted the comment.

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