def __init__(
        self, initial_block_size: int = 8, capacity_factor: float = 0.75
    ) -> None:
        self._initial_block_size = initial_block_size
        self._buckets: list[_Item | None] = [None] * initial_block_size
        assert 0.0 < capacity_factor < 1.0
        self._capacity_factor = capacity_factor
        self._len = 0