def for in yield or next for in

>>> a=['', 'a', 'a', '', 'a', '']
>>> b=['b', 'b', 'b']
>>> def f(s):
	for x in s:
		yield x

>>> [i or f(b).next() for i in a]
['b', 'a', 'a', 'b', 'a', 'b']
>>>