Append values corresponding to a unique label as a list of values in pandas dataframe

Question

I have got a dataframe:

df  =      A    B
       0   a    1
       1   b    2
       2   a    3 
       3   d    4

I want to update it like:

df  =      A    B
       0   a    [1, 3]
       1   b    [2] 
       2   d    [4]

Answer 1

You can groupby column A and convert the grouped elements in B to lists with apply :

df.groupby('A').B.apply(list).reset_index()

   A       B
0  a  [1, 3]
1  b     [2]
2  d     [4]