for j in range(2**i//2)

>>> def ml(n, e=''):
...     assert n > 0
...     if n == 1:
...         return [e + '0', e + '1']
...     return [ml(n - 1, e + '0'), ml(n - 1, e + '1')]
... 
>>> ml(1)
['0', '1']
>>> ml(2)
[['00', '01'], ['10', '11']]
>>> ml(3)
[[['000', '001'], ['010', '011']], [['100', '101'], ['110', '111']]]
>>>