Backward loop in Python

I need to loop backwards from i=n-2 to i = 0 to code this math formula:

for i in range(n-2,0):
    X[i] = Y[i]
    for m in range(i+1,n):

        X[i] = X[i] - T[i,m] * X[m]

It doesn’t work, what am I doing wrong?

All numbers from numpy arrays

>Solution :

if you want to loop backward you can use the for loop like following

range(start, end, step)

the step is 1 by default. in your case, you have to specify the decrement in order the loop the work.