You can use itertools.cycle:
>>> from itertools import cycle
>>> lis = [[10,13,17],[3,5,1],[13,11,12]]
>>> cyc = cycle((-1, 1))
>>> 50 + sum(x*next(cyc) for x in lis[0]) # lis[0] is [10,13,17]
36
Here the generator expression inside sum would return something like this:
>>> cyc = cycle((-1, 1))
>>> [x*next(cyc) for x in lis[0]]
[-10, 13, -17]
You can also use zip here:
>>> cyc = cycle((-1, 1))
>>> [x*y for x, y in zip(lis[0], cyc)]
[-10, 13, -17]