怎么写出上下三角矩阵？

1
12
123
1234
12345
123456
的那种下三角矩阵，还有上三角的

T = int(input())
for i in range(T):
    t = int(input())
    s = []
    s1 = 0
    s2 = 0
    for j in range(t):
        s.append(list(map(int,input().split())))
    for j in range(t):
        for k in range(t):
            if k > j:
                s1 += s[j][k]
            elif k < j:
                s2 += s[j][k]
    if s1 == 0 and s2 != 0:
        print("lower triangular matrix")
    elif s1 != 0 and s2 == 0:
        print("upper triangular matrix")
    else:
        print("no")