Create list with combinations of 3 elements of other list with repetitions

Question

I have a list with some elements and I want to create a list where I combine this items 3 by 3. Until now I managed to do this but I can't figure how can I do combinations with repeated elements. So far the code I have is this one:

list_of_aa = ["A", "R", "N", "D", "C", "Q", "E", "G", "H", "I", "L", "K", "M", "F", "P", "S", "T", "W", "Y", "V"]

combinations = list(itertools.combinations(list_of_aa, 3))

But with this code I'm missing triplets like "AAA" or "MAA". Do you have any idea how can I solve this problem? Thanks in regard.

Answer 1

You want to have the cartesian product of your items, so use itertools.product():

itertools.product(list_of_aa, repeat=3)

Note that this produces len(list_of_aa) ** 3 (so the cube) number of elements, you may want to avoid materialising that all into a list. For your sample input, 8000 3-element tuples are produced.

Demo taking slices out of the whole result:

>>> from itertools import product, islice
>>> list_of_aa = ["A", "R", "N", "D", "C", "Q", "E", "G", "H", "I", "L", "K", "M", "F", "P", "S", "T", "W", "Y", "V"]
>>> for combo in islice(product(list_of_aa, repeat=3), 5):
...     print(''.join(combo))
...
AAA
AAR
AAN
AAD
AAC
>>> for combo in islice(product(list_of_aa, repeat=3), 1000, 1005):
...     print(''.join(combo))
...
NLA
NLR
NLN
NLD
NLC
>>> for combo in islice(product(list_of_aa, repeat=3), 2000, 2005):
...     print(''.join(combo))
...
QAA
QAR
QAN
QAD
QAC