创建新列时 Python Pandas SettingWithCopyWarning

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?

Answer 1

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, ie:

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