I'm confused by this pandas warning. I'm already using the recommended .loc[,] format, but the warning persists. Can someone explain why this warning is appearing?
df = pd.DataFrame({'A': [1, 2, 3], 'B': [4, 5, 6]})
sub_df = df.loc[df.A > 1]
sub_df.loc[:, 'C'] = sub_df.A + sub_df.B
Here is the warning message:
<ipython-input-203-8af37ac9de96>:3: 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_guide/indexing.html#returning-a-view-versus-a-copy
sub_df.loc[:, 'C'] = sub_df.A + sub_df.B
The SettingWithCopyWarning when using .loc, occurred because you are modifying a copy of a DataFrame.
And you didn't use the view of the original. So this warning is designed to alert you to the possibility that your changes may not be reflected in the original DataFrame as you might intend.
So you have to explicitly create copy of the slice. It also tells pandas that you are intentionally working with a new DataFrame and silences the warning.
sub_df = df.loc[df['A'] > 1].copy()
sub_df.loc[:, 'C'] = sub_df['A'] + sub_df['B']