deduper with history of duplicates and support for hashable types

deduper.py

from collections import deque


def dedupe_with_history(items, key=None, history=None):
    previous = deque(maxlen=history)
    seen = set()
    for item in items:
        val = item if key is None else key(item)
        if val not in seen:
            yield item
            seen.add(val)
        else:
            previous.append(item)
    yield previous


"""
>> list(dedupe_with_history([5, 1, 1, 3, 66, 8, 66])
[5, 1, 3, 66, 8, deque([1, 66])]

>> *items, list(dupes) = dedupe_with_history([5, 1, 1, 3, 66, 8, 66])
>> items
[5, 1, 3, 66, 8]
>> dupes
[1, 66]
"""