Insert rows as a result of a groupby operation into the original dataframe

Question

For example, I have a pandas dataframe as follows:

col_1   col_2   col_3  col_4
a       X        5      1
a       Y        3      2
a       Z        6      4         


        

Answer 1

If you can guarantee that X and Z appear only once in a group, you can use a groupby and pd.concat operation:

new = df[df.col_2.isin(['X', 'Z'])]\
      .groupby(['col_1'], as_index=False).sum()\
      .assign(col_2='NEW')

df = pd.concat([df, new]).sort_values('col_1')

df
  col_1 col_2  col_3  col_4
0     a     X      5      1
1     a     Y      3      2
2     a     Z      6      4
0     a   NEW     11      5
3     b     X      7      8
4     b     Y      4      3
5     b     Z      6      5
1     b   NEW     13     13