Remove duplicates while preserving original order (set() loses order on collisions). Optional key function lets you dedupe by a derived value — e.g., lowercased email.
Python
from typing import Callable, Hashable, Iterable, Iterator, TypeVar
T = TypeVar("T")
def unique_everseen[T](items: Iterable[T], key: Callable[[T], Hashable] | None = None) -> Iterator[T]:
seen: set[Hashable] = set()
for item in items:
k = key(item) if key else item # type: ignore[arg-type]
if k not in seen:
seen.add(k)
yield item
list(unique_everseen([1, 2, 1, 3, 2, 4])) # [1, 2, 3, 4]
emails = ["Bob@X.com", "alice@x.com", "BOB@x.com"]
list(unique_everseen(emails, key=str.lower)) # ['Bob@X.com', 'alice@x.com']Save your own code snippets
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