sum.py

from bisect import bisect_left, bisect_right


def pairs_sums_to(k, xs):
    xs = [*sorted(xs)]
    lo, hi = 0, len(xs) - 1
    while True:
        left_pos = bisect_left(xs, k, lo, hi)
        if left_pos > hi or left_pos == lo:
            return
        s1 = xs[left_pos]
        s2 = k - s1
        lo = bisect_right(xs, s2, lo, hi)
        hi = left_pos - 1
        if xs[lo - 1] == s2:
            yield s1, s2


print(list(pairs_sums_to(10, [0, 1, 2, 3, 4, 5, 5, 5, 6, 7, 8, 9, 10, 11])))