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- from collections import defaultdict
- from typing import Callable, Collection, Dict, List, TypeVar
- __all__ = ["group_by"]
- K = TypeVar("K")
- T = TypeVar("T")
- def group_by(items: Collection[T], key_fn: Callable[[T], K]) -> Dict[K, List[T]]:
- """Group an unsorted collection of items by a key derived via a function."""
- result: Dict[K, List[T]] = defaultdict(list)
- for item in items:
- key = key_fn(item)
- result[key].append(item)
- return result
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