gistfile1.txt

DATA = [
    [1, 2, 3],
    [4, [5, 6], 7],
    [[8, 9], 10, [11, 12, 13, 14]],
    15,
    [16, 17, [18, 19], 20]
    ]

def walk(data):
    for el in data:
        if isinstance(el, (list, tuple)):
            for sub_el in walk(el):
                yield sub_el
        else:
            yield el

for i in walk(DATA):
    print i


output:
$ python test_breadth.py
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20