Хитрое заполнение

n, m = 4, 5
a = [ [0] * m for i in range(n) ]

i = j = 0
k = 1
while k <= n * m:
    if i < n and j < m:
        a[i][j] = k
        k += 1
    if j == 0:
        i, j = 0, i + 1
    else:
        i += 1
        j -= 1


for row in a:
    print(*row, sep='\t')