>>> def flat(lst):
    i=0
    while i<len(lst):
        while True:
            try:
                lst[i:i+1] = lst[i]
            except (TypeError, IndexError):
                break
        i += 1

>>> lst = [[1], 2, [[3,4], 5], [[[]]], [[[6]]], 7, 8, []]
>>> flat(lst)
>>> lst
[1, 2, 3, 4, 5, 6, 7, 8]