Pandas show group sum on all rows [duplicate]

Question

This question already has an answer here:

• Best Way to add group totals to a dataframe in Pandas 1 answer

Given the following dataframe:

col_a   |   col_b_tosum   
 b      |    5 
 b      |    5
 b      |    1
 c      |    6
 c      |    3
 a      |    2
 a      |    2

I would like to show the sum of each col_ group on all rows, like this:

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

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