How reshape pandas dataframe side by side based on a column?

Question

I am just trying to do the following exercise: I want to use the values of column A as an aggregator for Column B, but instead of using a function like "sum" or "mean", I want to have the values side by side. let me give you an example.

Before:

d = {'A': [0,1,2,3,4,0,1,2,3,4],
     'B': [10,11,17,13,15,2,3,19,20,21]}
df = pd.DataFrame(d)
df

A   B
0   10
1   11
2   17
3   13
4   15
0   2
1   3
2   19
3   20
4   21

How I would like to see it:

A   D   E
0   10  2
1   11  3
2   17  19
3   13  20
4   15  21

In my head, the solution would be using unstack or maybe a groupby, but I can't think of how to formulate it.

Thanks