[
[1],
[2,3],
[4,5,6]
]

[1, 2, 4, 1, 3, 5, 1, 2, 6]

a=[
    [2,3],
    [1],
    [4,5,6]
    ]
def flatten(lst):
    l = sorted(lst, key=lambda x: len(x))
    m = len(l[-1])
    l = map(iter, l)
    for s in xrange(m):
        for x in l:
            try:
                yield next(x)
            except StopIteration:pass
for i in flatten(a):
    print i

l =[
[1],
[2,3],
[4,5,6]
]
res = []
for i in range(len(l)):
    for j in range(len(l[-1])):
        try:
            res.append(l[j][i])
        except IndexError:
            res.append(l[j][0])
            
res
Out[4]: [1, 2, 4, 1, 3, 5, 1, 2, 6]

from itertools import cycle
def foxy_iter(ls):
    n = max(len(x) for x in ls)
    nls = [len(x) == n and iter(x) or cycle(x) for x in ls]
    for _ in range(n):
        for x in nls:
            yield next(x)
print list(foxy_iter([
    [1],
    [2,3],
    [4,5,6]
    ]
))

class RingList(object):
    def __init__(self, lst):
        self.__lst = lst
        self.index = -1
    def __iter__(self):
        return self
    def next(self):
        self.index += 1
        try:
            return self.__lst[self.index]
        except IndexError:
            self.index = 0
            return self.__lst[self.index]
a = [
    [2, 3],
    [1],
    [4, 5, 6]
    ]
def flatten(lst):
    l = sorted(lst, key=lambda x: len(x))
    m = len(l[-1])
    l = map(RingList, l)
    for _ in xrange(m):
        for x in l:
            yield next(x)
flatten(a)
for i in flatten(a):
    print i

nls = [len(x) == n and iter(x) or cycle(x) for x in ls]

nls = [cycle(x) for x in ls]