def pairs(ints, target, result=None):
    if result is None:
        result = []
    first = ints.pop(0)
    remain = target - first
    if remain in ints:
        result.append((first, remain))
        ints.remove(remain)
    if (len(ints) > 1):
        pairs(ints, target, result)
    return result


pairs([1, 2, 3, 4], 5)       # [(1, 4), (2, 3)]
pairs([1, -2, 5, 4, 7], 5) # [(1, 4), (-2, 7)]