python矩阵运算,求一个3*3矩阵主对角线元素之和

#矩阵运算
#求一个3*3矩阵主对角线元素之和
a = []
sum = 0.0
for i in range(3):
    a.append([])
    for j in range(3):
        a[i].append(float(input("number:")))
for i in range(3):
    sum += a[i][i]
print(a)
print(sum)