A nice approach to this problem is to use defaultdict. As @Martin already said, list comprehension is not the right tool to produce two lists. Using defaultdict would enable you to create segregation using a single iteration. Moreover your code would not be limited in any form.
>>> from collections import defaultdict
>>> s=['A','B','A','A','A','B','B']
>>> listOf = defaultdict(list)
>>> for idx, elem in enumerate(s):
listOf[elem].append(idx)
>>> listOf['A'], listOf['B']
([0, 2, 3, 4], [1, 5, 6])
