Add 2 new columns to existing dataframe using apply

anky_91's answer highlights a useful option in apply.

For this particular case however, apply is not even required,

df['add'], df['multiply'] = add_multiply(df['col1'],df['col2'])

You can add result_type='expand' in the apply:

‘expand’ : list-like results will be turned into columns.

df[['add', 'multiply']]=df.apply(lambda x: add_multiply(x['col1'], x['col2']),axis=1,
                             result_type='expand')

Or call a dataframe constructor:

df[['add', 'multiply']]=pd.DataFrame(df.apply(lambda x: add_multiply(x['col1'], 
                                    x['col2']), axis=1).tolist())

   col1  col2  add  multiply
0     1     3    4         3
1     2     4    6         8

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