Generally in Python it's much better to use list comprehensions when possible, it offers significant speedups usually: def zip(ls1, ls2): return [(ls1[

Generally in Python it's much better to use list comprehensions when possible, it offers significant speedups usually: def zip(ls1, ls2): return [(ls1[i], ls2[i]) for i in range(min(len(ls1), len(ls2)))] And indeed you could even use a generator instead: def zip(ls1, ls2): return ((ls1[i], ls2[i]) for i in range(min(len(ls1), len(ls2))))