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from itertools import filterfalse
from typing import (
Callable,
Iterable,
Iterator,
Optional,
Set,
TypeVar,
Union,
)
# Type and type variable definitions
_T = TypeVar('_T')
_U = TypeVar('_U')
def unique_everseen(
iterable: Iterable[_T], key: Optional[Callable[[_T], _U]] = None
) -> Iterator[_T]:
"List unique elements, preserving order. Remember all elements ever seen."
# unique_everseen('AAAABBBCCDAABBB') --> A B C D
# unique_everseen('ABBCcAD', str.lower) --> A B C D
seen: Set[Union[_T, _U]] = set()
seen_add = seen.add
if key is None:
for element in filterfalse(seen.__contains__, iterable):
seen_add(element)
yield element
else:
for element in iterable:
k = key(element)
if k not in seen:
seen_add(k)
yield element
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