Given a sequence of pairs (A, B), return a list of 3-tuples (A, B, T) where T is a boolean that marks whether the sequence contains the inverted pair (B, A) as well.

proc.py

from typing import Sequence

def mark_bidirectionals(pairs: Sequence) -> list:
    """Given a sequence of pairs (A, B), return a list of 3-tuples (A, B, T) where T is a boolean that
    marks whether the sequence contains the inverted pair (B, A) as well. Order is preserved, but all 
    identical and inverted pairs to the right of (i.e. on higher indexes than) the original pair are removed.
    Implementation note: pairs must be immutable (usable as dict keys)."""
    marks = dict()
    for a, b in pairs:
        if (b, a) in marks:
            marks[(b, a)] = True
        elif (a, b) not in marks:
            marks[(a, b)] = False
    return list((a, b, t) for ((a, b), t) in marks.items())

test_proc.py

import pytest
from proc import mark_bidirectionals


def test_mark_bidirectionals_empty():
    r = mark_bidirectionals([])
    assert r == []


def test_mark_bidirectionals_simple():
    r = mark_bidirectionals(
        [("A", "B"), ("C", "D"), ("E", "F"), ("B", "A"), ("G", "H"), ("J", "K")])
    assert r == [("A", "B", True), ("C", "D", False),
                 ("E", "F", False), ("G", "H", False), ("J", "K", False)]


def test_mark_bidirectionals_multi():
    r = mark_bidirectionals([("A", "B"), ("C", "D"), ("E", "F"),
                            ("B", "A"), ("B", "A"), ("G", "H"), ("B", "A"), ("J", "K")])
    assert r == [("A", "B", True), ("C", "D", False),
                 ("E", "F", False), ("G", "H", False), ("J", "K", False)]


def test_mark_bidirectionals_identical_dups_removed():
    r = mark_bidirectionals(
        [("A", "B"), ("C", "D"), ("E", "F"), ("B", "A"), ("A", "B"), ("G", "H"), ("G", "H"), ("J", "K")])
    assert r == [("A", "B", True), ("C", "D", False),
                 ("E", "F", False), ("G", "H", False), ("J", "K", False)]


def test_mark_bidirectionals_bi_dups_removed():
    r = mark_bidirectionals(
        [("A", "B"), ("C", "D"), ("E", "F"), ("B", "A"), ("B", "A"), ("G", "H"), ("B", "A"), ("J", "K")])
    assert r == [("A", "B", True), ("C", "D", False),
                 ("E", "F", False), ("G", "H", False), ("J", "K", False)]