def deepdict():
    return collections.defaultdict(deepdict)

def deepdictify(tupdict):
    res = deepdict()
    for key, val in tupdict.items():
    	tempres = res
    	for kpart in key[:-1]:
    	    tempres = tempres[kpart]
    	tempres[key[-1]] = val

@attr.s
class Book(object):
    shop_label = attr.ib()
    cell_label = attr.ib()
    book_id = attr.ib()
    count = attr.ib()

def build_book_inventory(book_ids, shops):
    shop_labels = [shop['label'] for shop in shops]
    book_list = Persistency.books_table.read(
	shops=shop_labels,
	books=book_ids)
    inventory = {}
    for item in book_list:
    	key = tuple(map(functools.partial(operator.getitem, item),
    	                ('shop_label', 'cell_label', 'book_id')))
	inventory[key] = item['count']
    return deepdictify(inventory)