Sorted by speed:
list_of_lists = [[x,1] for x in xrange(1000)]
%timeit list(itertools.chain(*list_of_lists))
100000 loops, best of 3: 14.6 µs per loop
%timeit list(itertools.chain.from_iterable(list_of_lists))
10000 loops, best of 3: 60.2 µs per loop
min(timeit.repeat("ll=[];\nfor l in list_of_lists:\n ll.extend(l)", "list_of_lists=[[x,1] for x in xrange(1000)]",repeat=3, number=100))/100.0
9.620904922485351e-05
%timeit [y for z in list_of_lists for y in z]
10000 loops, best of 3: 108 µs per loop
%timeit sum(list_of_lists, [])
100 loops, best of 3: 3.7 ms per loop
