Контрольная. Задание 4

from collections import Counter


with open("input.txt", "r", encoding="utf-8") as file:
    buys_count = Counter()
    buys = [line.strip().split(",") for line in file.readlines()]

    for buy in buys:
        buys_pairs = []
        buy.sort()

        for i in range(len(buy)):
            for j in range(i, len(buy)):
                if buy[i] != buy[j]:
                    buys_pairs.append((buy[i], buy[j]))

        buys_count.update(buys_pairs)

    max_count = buys_count.most_common(1)[0][1]
    max_buy = None

    for buy in buys_count.most_common():
        if buy[1] == max_count:
            if max_buy == None or buy[0] <= max_buy:
                max_buy = buy[0]

    print(",".join(max_buy))