find itens which has odd frequency

find_odds.py

from collections import Counter


def find_odds(seq):
    return [k for k, v in Counter(seq).items() if v % 2 == 1]


print(find_odds([20, 1, -1, 2, -2, 3, 3, 5, 5, 1, 2, 4, 20, 4, -1, -2, 5]))

Using set only:

def find_odds(seq):
    odds = set()
    for v in seq:
        if v in odds:
            odds.remove(v)
        else:
            odds.add(v)
    return odds


print(find_odds([20, 1, -1, 2, -2, 3, 3, 5, 5, 1, 2, 4, 20, 4, -1, -2, 5]))