Returns a list containing only unique elements from original list, with an optional key function

ordered_set.py

from typing import Iterable, Callable, List

def ordered_set(iterable: Iterable, key: Callable=lambda x: x) -> List:
    """
    Returns a new list containing all unique items from the iterable. A custom key function can
    be specified to customize the uniqueness constraint, for example to return the first unique
    element from the input list based on the second item in each element, one could pass
    `lambda x: x[1]`.
    """
    out_list = []
    added = set()
    for val in iterable:
        if not key(val) in added:
            out_list.append(val)
            added.add(key(val))
    return out_list