Accessing the instance from which an instance is referenced in Python

Question

There probably is a name for what I'm asking for, perhaps it's best shown through an example. Suppose I have these classes:

class MonkeyCage(object):
    def __init__(self, dimensions, monkeys=None):
        self.dimensions = dimensions
        self.monkeys = monkeys

    def add_monkey(self, monkey):
        self.monkeys.append(monkey)


class Monkey(object):
    def __init__(self, name):
        self.name = name

Suppose I create a zoo, with a MonkeyCage and a first Monkey:

>>> dimensions = {'width': 100, 'height': 200, 'depth': 70}
>>> cage = MonkeyCage(dimensions) 
>>> monkey = Monkey("LaundroMat")

and add the monkey to the MonkeyCage:

>>> cage.add_monkey(monkey)

Is there a way -- via the monkey instance -- to get the dimensions of the cage the monkey was added to?

as you suggested, this kind of entity relationship (one to many) is very common. Normally a foreign key is used to link the monkey to the cage. It is used in RDBMS (Relational DataBase Systems) to link object instances (represented by rows in a table, which in turn represents the object class. You'd have a table for the Monkey and one for the Cage). — Pynchia
– Pynchia, Commented May 10, 2015 at 8:47
I regularly use ORMs and was indeed wondering whether a similar concept to foreign keys was doable :) — Mathieu Dhondt
– Mathieu Dhondt, Commented May 12, 2015 at 10:35

Tasos Vogiatzoglou · Accepted Answer · 2015-05-10 08:04:07Z

2

You'll have to pass the parent somehow on monkey. E.g. in add_monkey, you could set set monkey.parent=self and access it that way.

answered May 10, 2015 at 8:04

Tasos Vogiatzoglou

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2 Comments

Mathieu Dhondt Over a year ago

Is there any danger in class instances referencing each other (memory-wise or other)?

Tasos Vogiatzoglou Over a year ago

@LaundroMat, yes, you create cycles which the reference counting (the default garbage collection strategy) cannot handle. There is an optional garbage collector though that can resolve this. Check docs.python.org/2/library/gc.html

Stephen Lin · Accepted Answer · 2015-05-10 14:56:10Z

1

Just give monkey a tag when you add it to that cage.

class MonkeyCage(object):
    def __init__(self, dimensions, monkeys=None):
        self.dimensions = dimensions
        self.monkeys = monkeys

    def add_monkey(self, monkey):
        monkey.cage_dimension = self.dimensions
        self.monkeys.append(monkey)


class Monkey(object):
    def __init__(self, name):
        self.name = name

Then you can get info about cage through monkey.cage_dimensions

answered May 10, 2015 at 14:56

Stephen Lin

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