How to pass an item reference to the Payment.get_purchased_items method?

[rtdev-com]

Can someone help me understand how to pass some sort of item reference to Payment.get_purchased_items?

1. The get_purchased_items method does not take any input arguments except for self
2. The BasePayment class does not have any item references either

    def get_purchased_items(self) -> Iterable[PurchasedItem]:
         # do we have item info here?
         item = get_item_from_somewhere()

        # Return items that will be included in this payment.
        yield PurchasedItem(
            name=item.name,
            sku=item.sku,
            quantity=1,
            price=item.price,
            currency='USD',
        )

This part is unclear to me from reading the documentation since the docs just hard code an item.