""" write a loop that adds up the numbers from 1 to n """

def loop_invariant(i, total):
    # the sum so far is i*(i+1)/2 which is 1+2+3+...i
    # for each i as we loop from 1 to n
    total_should_be = i * (i+1) // 2
    if total != total_should_be:
        print("OOPS : loop invariant failed ")

n = 10
total = 0
for i in range(1, 1+n):
    total = total + i
    loop_invariant(i, total)  # check to see that our partial result is OK
print("total of 1+2+...+10 is ", total)