fill the matrix

"""
fill a square matrix in the following way:

0   20  19  17  14
1   0   18  16  13
2   5   0   15  12
3   6   8   0   11
4   7   9   10  0
"""

size = 5
# init matrix
matrix = [[None for _ in range(size)] for _ in range(size)]

a = 0
b = size ** 2 - (size - 1)

for i in range(size):
    for j in range(size):
        if i == j:
            matrix[i][j] = 0
        elif j > i:
            a += 1
            matrix[i][j] = a
        else:
            b -= 1
            matrix[i][j] = b


# helper function for printing
def print_matrix(m):
    dim = len(m)
    buffer = []
    for c in range(dim):
        for r in range(dim):
            buffer.append(matrix[r][c])
        print(*["{:3}".format(x) for x in buffer])
        buffer = []


print_matrix(matrix)