Should objects know of the objects they're used in? - oop

Question

class Item:
def __init__(self, box, description):
self._box = box
self._description = description
class Box:
def __init__(self):
self.item_1 = Item(self, 'A picture')
self.item_2 = Item(self, 'A pencil')
#etc
old_stuff = Box()
print(old_stuff.item_1.box.item_1.box.item_2.box.item_1)
Above is shown an example piece of code which demonstrates my problem better than I ever could with plain text. Is there a better way to find in what box something is? (In what box is the picture?) Since I am not particularly fond of the above solution because it allows for this weird up and down calling which could go on forever. Is there a better way to solve this problem or is this just a case of: If it's stupid and it works, it ain't stupid.
Note: this trick isn't python specific. It's doable in all object-oriented programming laguages.

Answer 1

There is no right or wrong way to do this. The solution depends on how you want to use the object.
If your use-case requires that an item know in which box it is stored, then you need a reference to the box; if not, then you don't need the association.
Similarly, if you need to which items are in a given box, then you need references to the items in the box object.
The immediate requirement (that is, the current context) always dictates how one designs a class model; for example, one models an item or a box differently in a UI layer from how one would model it in a service layer.

Answer 2

You must introduce new class - ItemManager or simply dict or other external structure to store information about which box contain your item:
class Item:
def __init__(self, description):
self.description = description
class Box:
def __init__(self, item_1, item_2):
self.item_1 = item_1
self.item_2 = item_2
class ItemManager:
def __init__(self):
self.item_boxes = {}
def register_item(self, item, box):
self.item_boxes[item] = box
def deregister_item(self, item):
del self.item_boxes[item]
def get_box(self, item):
return self.item_boxes.get(item, None)
item_manager = ItemManager()
item_1 = Item("A picture")
item_2 = Item("A pencil")
item_3 = Item("A teapot")
old_stuff = Box(item_1, item_2)
item_manager.register_item(item_1, old_stuff)
item_manager.register_item(item_2, old_stuff)
new_stuff = Box(item_3, None)
item_manager.register_item(item_3, new_stuff)
box_with_picture = item_manager.get_box(item_2)
print box_with_picture.item_1.description
Also see SRP: an item should not know which box contains it.