Pandas show group sum on all rows [duplicate]

Answer 1

Use groupby with transform:

df['group_sum'] = df.groupby('col_a')['col_b_tosum'].transform('sum')

Output:

  col_a  col_b_tosum  group_sum
0     b            5         11
1     b            5         11
2     b            1         11
3     c            6          9
4     c            3          9
5     a            2          4
6     a            2          4