Creating custom combination from two lists - python-3.5

Question

I am looking to use two lists:
L1 = ['a', 's', 'd']
L2 = [str(1), str(2)]
I need to create a third list:
L3 = [(a1, s1, d1), (a1, s1, d2), ... ]
L3 has tuples of size 3 where each tuple has only non-repetitive elements from L1 but can have repetitive elements from L2.
i.e. a pair as (a1, s2, d2) is allowed but (a1, a2, d1) is not allowed.
L3 has tuples of size 3.
I am working with large L1 and L2 so the above example is only for illustration. I am not sure how to approach this problem. I have thought about using itertools permutation and combination modules but I am not getting the list L3 above. One brute force solution is to do something as:
L3 = list(itertools.combinations(list(itertools.product(L1, L2)), 3))
and then condition out the elements as ('a', '1'), ('a', '2'), ('d', '2') but for a large combination that is not efficient for loop.

Answer 1

Since it sounds like you want L1 to be the first elements of the tuples, I think we simply need to zip them, not iter-anything them. We only need to take the product of L2.
In [327]: [list(zip(L1, p)) for p in itertools.product(L2, repeat=len(L1))]
Out[327]:
[[('a', '1'), ('s', '1'), ('d', '1')],
[('a', '1'), ('s', '1'), ('d', '2')],
[('a', '1'), ('s', '2'), ('d', '1')],
[('a', '1'), ('s', '2'), ('d', '2')],
[('a', '2'), ('s', '1'), ('d', '1')],
[('a', '2'), ('s', '1'), ('d', '2')],
[('a', '2'), ('s', '2'), ('d', '1')],
[('a', '2'), ('s', '2'), ('d', '2')]]
where you can replace [ and ] with ( and ) to turn the listcomp into a genexp if you don't want to materialize the whole object at once.
If you want to merge your tuples' elements into one string, you could do that too:
In [338]: gen = (tuple(''.join(pair) for pair in zip(L1, p))
for p in itertools.product(L2, repeat=len(L1)))
In [339]: for elem in gen:
...: print(elem)
('a1', 's1', 'd1')
('a1', 's1', 'd2')
('a1', 's2', 'd1')
('a1', 's2', 'd2')
('a2', 's1', 'd1')
('a2', 's1', 'd2')
('a2', 's2', 'd1')
('a2', 's2', 'd2')