def unique_items(seq):
    """A function that returns all unique items of a list in an order-preserving way"""
    id_fn = lambda x: x
    seen_items = {}
    result = []
    for item in seq:
        marker = id_fn(item)
        if marker in seen_items: continue
        seen_items[marker] = 1
        result.append(item)
    return result