>>> list(itertools.product([0,1,2],[3,4,5]))
[(0, 3), (0, 4), (0, 5), (1, 3), (1, 4), (1, 5), (2, 3), (2, 4), (2, 5)]

>>> my_product([0,1,2], [3,4,5])
[(0, 3),                 # element 0 from both iterables
 (0, 4), (1, 3),         # elements 0,1 and 1,0 (sums to 1)
 (0, 5), (1, 4), (2, 3), # elements 0,2 and 1,1 and 2,0 (sums to 2)
 (1, 5), (2, 4),         # elements 1,2 and 2,1 (sums to 3)
 (2, 5)]                 # elements 2,2 (sums to 4)