Find unique pairs in list of pairs

Question

I have a (large) list of lists of integers, e.g.,

a = [
    [1, 2],
    [3, 6],
    [2, 1],
    [3, 5],
    [3, 6]
    ]

Most of the pairs will appear twice, where the order of the integers doesn't matter (i.e., [1, 2] is equivalent to [2, 1]). I'd now like to find the pairs that appear only once, and get a Boolean list indicating that. For the above example,

b = [False, False, False, True, False]

Since a is typically large, I'd like to avoid explicit loops. Mapping to frozensets may be advised, but I'm not sure if that's overkill.

Answer 1

ctr = Counter(frozenset(x) for x in a)
b = [ctr[frozenset(x)] == 1 for x in a]

We can use Counter to get counts of each list (turn list to frozenset to ignore order) and then for each list check if it only appears once.