Замена цикла на генератор:

n = 4
a = [0] * n
a = [[2] * i + [1] + [0] * (n - i - 1) for i in range(n)]
for row in a:
    print(' '.join([str(elem) for elem in row]))

http://pythontutor.ru/lessons/2d_arrays/

>>> n = 4
>>> a = [0] * n
>>> a = [[2] * i + [1] + [0] * (n - i - 1)
     for i in range(n)]
>>> for row in a:
     print(' '.join([str(elem) for elem in row]))

     
1 0 0 0
2 1 0 0
2 2 1 0
2 2 2 1