python - Get all pairwise combinations from a list

Question

For example, if the input list is

[1, 2, 3, 4]

I want the output to be

[(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]

If possible, I would like a solution which is better than the brute force method of using two for loops. How do I implement this?

Answer 1

Though the previous answer will give you all pairwise orderings, the example expected result seems to imply that you want all unordered pairs.

This can be done with itertools.combinations:

>>> import itertools
>>> x = [1,2,3,4]
>>> list(itertools.combinations(x, 2))
[(1, 2), (1, 3), (1, 4), (2, 3), (2, 4), (3, 4)]

Compare to the other result:

>>> list(itertools.permutations(x, 2))
[(1, 2), (1, 3), (1, 4), (2, 1), (2, 3), (2, 4), (3, 1), (3, 2), (3, 4), (4, 1), (4, 2), (4, 3)]