Python の yield from をつかって flatten する ref: http://qiita.com/trsqxyz/items/013ea4ece0de52328f17

file0.txt

def flatten(x):
    if hasattr(x, '__iter__') and not isinstance(x, str):
        for y in x:
            yield from flatten(y)
    else:
        yield x

file1.txt

In : ls = [1,2,3,[4,5],[6,[7,8,[9,10,11],12]]]

In : list(flatten(ls))
Out: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]