Python - Matrix Identity multiplication

I have a a dataframe a which is

a = [[3],
     [12],
     [15]]

and I want to turn it into

b = [[3, 0, 0],
     [0, 12,0],
     [0, 0, 15]]

It has been a while since high school and my matrix multiplication is a little off. Any help would be great.

Solution

You can use numpy functions

>>> import numpy as np
>>> import pandas as pd
>>> b = np.zeros((3,3), dtype=int)
>>> np.fill_diagonal(b, [3, 12, 15])
>>> b
array([[ 3,  0,  0],
       [ 0, 12,  0],
       [ 0,  0, 15]])

If you need a DataFrame

>>> pd.DataFrame(b)
   0   1   2
0  3   0   0
1  0  12   0
2  0   0  15