def global for in yield if else next for in

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

		
>>> [i if i!='' else f().next() for i in a]
['b', 'a', 'a', 'b', 'a', 'b']
>>>