The purpose is to create a program that generates 3x3 matrices, in which it asks me for the values of each matrix and then shows the add, subtract, multiply or exit menu, but for some reason the script does not run. I need help. I am using python 3.7

a = [[],[],[]] b = [[],[],[]] def multiply(a,b,i,j): return a[i][0]*b[0][j]+a[i][1]*b[1][j]+a[i][2]*b[2][j] for i in range(3): for j in range(3): a[i][j] = int(input("Enter the value of the {} row {} column of the first matrix: ".format(i+1,j+1))) b[i][j] = int(input("Enter the value of the {} row {} column of the second matrix:".format(i+1,j+1))) while True: print("MENU:") print("1: Sum") print("2: Subtraction") print("3: Multiplication") print("4: Exit") op= int(input("Please write the operation you want to execute")) if op == 1: c = [[],[],[]] for i in range(3): for j in range(3): c[i][j] = a[i][j]+b[i][j] print("a+b=:") print("{} {} {}".format(c[0][0],c[0][1],c[0][2])) print("{} {} {}".format(c[1][0],c[1][1],c[1][2])) print("{} {} {}".format(c[2][0],c[2][1],c[2][2])) if op == 2: c = [[],[],[]] for i in range(3): for j in range(3): c[i][j] = a[i][j]-b[i][j] print("a-b=:") print("{} {} {}".format(c[0][0],c[0][1],c[0][2])) print("{} {} {}".format(c[1][0],c[1][1],c[1][2])) print("{} {} {}".format(c[2][0],c[2][1],c[2][2])) c = [[],[],[]] for i in range(3): for j in range(3): c[i][j] = b[i][j]-a[i][j] print("b-a=:") print("{} {} {}".format(c[0][0],c[0][1],c[0][2])) print("{} {} {}".format(c[1][0],c[1][1],c[1][2])) print("{} {} {}".format(c[2][0],c[2][1],c[2][2])) if select == 3: c = ([[],[],[]]) for i in range(3): for j in range(3):  c[i][j]=multiply(a,b,i,j) print("a*b=:") print("{} {} {}".format(c[0][0],c[0][1],c[0][2])) print("{} {} {}".format(c[1][0],c[1][1],c[1][2])) print("{} {} {}".format(c[2][0],c[2][1],c[2][2])) c = [[],[],[]] for i in range(3): for j in range(3): c[i][j]=multiply(b,a,i,j) print("b*a=:") print("{} {} {}".format(c[0][0],c[0][1],c[0][2])) print("{} {} {}".format(c[1][0],c[1][1],c[1][2])) print("{} {} {}".format(c[2][0],c[2][1],c[2][2])) if select == 4: break