Objects containing other Objects in Python

Question

I was having a quiz in class, and one of the questions asserted that an object in Python cannot contain other objects in itself, any objects. It does not make sense, since what is the problem with having a class table, and a class house, when a house is instantiated, its constructor creates an object table as its private variable. Doesn't it mean that an object can contain other objects or am I missing something?

TIA!

The question (says C is incorrect):

Which of the following statements are correct?

A: A reference variable is an object.

B: A reference variable refers to an object.

C: An object may contain other objects.

D: An object can contain the references to other objects.

I assume, whoever is asking the question, want's you to understand the difference between actual value (object) and reference. hence (A) should be wrong, and (C) as well...because the reference to an object is not the object itself, and any object can only ever reference other objects (not contain them). (B) and (D) should be correct. — mrxra
– mrxra, Commented Jul 30, 2020 at 12:24

AKX · Accepted Answer · 2020-07-30 12:14:45Z

3

class Table:
    pass

class House:
    def __init__(self):
        self.dining_table = Table()
        self.bedside_table = Table()

will naturally work just fine.

I think the original question had two points to it:

Any old object might not be able to contain other objects; e.g. ints are objects too and can't contain other objects.
Technically no object in Python actually contains other objects, they just refer to them.

edited Jul 30, 2020 at 12:14

answered Jul 30, 2020 at 12:08

AKX

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5 Comments

andriy Over a year ago

i have updated the question, but I will have to delete the screenshot later, thanks!

superb rain Over a year ago

So you're saying Python is lying to me when it says that [0].__contains__(0) is True? :-)

AKX Over a year ago

@superbrain __containsreferenceto__ is a little unwieldy... :)

superb rain Over a year ago

Right :-). Also, I think [x].__containsreferenceto__(y) should return False if x and y are equal but not the same object.

AKX Over a year ago

Yeah, if there was __containsreferenceto__ it would probably use is equality.

JPI93 · Accepted Answer · 2020-07-30 12:22:06Z

Given your original post I think that whoever wrote it is trying to get you to think about the difference between objects and references within the context of a containing object.

Technically, container objects do not actually contain other objects within Python, rather they contain references to other objects (see this document for reference). A simple example of this would be the tuple in this example:

x = ('hello', 'world')

This tuple object does not technically contain two str objects, it actually contains references to two str objects. This can be a bit confusing as people (generally) think of and treat objects in a similar way to references within the context of programming in Python - however it is important to understand the difference between the two as not being aware can lead to bugs, frustration and code that does not perform as desired.

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