how to split an array homogene in half of size by taking the triagle?

i have this array and i want to split it in the half how i can do it?

a = [[0., 15., 19., 18., 17.],
     [15., 0., 14., 12., 23.],
     [19., 14.,  0., 14., 21.],
     [18., 12., 14., 0., 14.],
     [17., 23., 21., 14.,  0.]]

how i can get this half size of this array:

[[0.],
    [15.,0],
    [19., 14.,0],
    [18., 12., 14.,0],
    [17., 23., 21., 14.,0]]

>Solution :

You can do something like this :

half = [row[:i+1] for i, row in enumerate(a)]

If you do not want the diagonal, you can add [1:] :

half = [row[:i+1] for i, row in enumerate(a[1:])]