What is the Pythonic way of summing the product of all combinations in a given list, such as:

[1, 2, 3, 4]
--> (1 * 2) + (1 * 3) + (1 * 4) + (2 * 3) + (2 * 4) + (3 * 4) = 35

(For this example I have taken all the two-element combinations, but it could have been different.)

Use itertools.combinations

>>> l = [1, 2, 3, 4]
>>> sum([i*j for i,j in list(itertools.combinations(l, 2))])
35