How to have an object in a class point to its class object in Python?

Question

I want to make a class where one object in that class belongs to a different type of class (though not in a nested way), and the second class object can point back to the class object that contains it.

That's pretty abstract. Like, let's say I want a class called "Party", which contains objects of the class Character. Let's say that Party has an int object called Total_gold. I know I could create that using something like:

class Party(object):
    def __init__(self, G1, c1, c2, ...other_stuff_goes_here):
        self.Total_gold = G1;
        self.char1 = Character(c1);
        self.char2 = Character(c2);
        # more stuff to initiate Party

class Character(object):
    def __init__(self, other_stuff_goes_here):
        # stuff to initiate Character

Now, let's say I have a function that takes in a Character object, but I want it to alter the Party that contains that Character. For example, let's say I want to have a function, Spend, that decreases the Party's Total_gold. Is there a way for Spend to take in a Character object, and then access the Party the Character is in?
Something like:

def Spend(character, amount): 
    #reduces the Total_gold of the party the character belongs to.
    character.Party.Total_gold -= amount

If so, how would I do this? I assume I need to put something in the __init__ function, but I don't know how I would do that. Python doesn't have pointers, right?

If another post has covered this, please let me know. I wasn't sure how to even ask this.

Thank you.

One option is to make each character store their own gold. Then make Party.Total_gold a @property that sums all the characters' gold. — 001
– 001, Commented Jan 5, 2021 at 4:33
From OO perspective this is a violation of Dependency Inversion Principle (DIP). Making a class Character associate to another class Party (because a Party has Character) is a violation of DIP — Aaj Kaal
– Aaj Kaal, Commented Jan 5, 2021 at 5:01
I like your idea about using @property; I didn't know those existed, this taught me something useful. Thank you. But, let's say I also needed a way to check if amount was less than Party.Total_gold (so that a Character can spend more gold than it has, but cannot spend more gold than the Party has). That's not possible to do with @property, right? — Simon
– Simon, Commented Jan 6, 2021 at 14:31

selplacei · Accepted Answer · 2021-01-05 04:47:25Z

1

Python objects don't internally store any information about being a member of another object, so you'll need to store the instance of Party inside of both Characters. The simplest way to do this is to pass self to Character.__init__:

class Character:
    def __init__(self, party, other_arg):
        self.party = party
        # Other stuff goes here

class Party:
    def __init__(self, c1, ...):
    self.char = Character(self, c1)

Then:

def spend(character, amount):
    character.party.total_gold -= amount

answered Jan 5, 2021 at 4:47

selplacei

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4 Comments

Aaj Kaal Over a year ago

Please delete this. This is 100% incorrect design. Either A has B or B has A but both is not a good design.

selplacei Over a year ago

My answer achieves exactly what the question is asking. Having circular references is a sign of bad design, but the implementation itself doesn't matter. If I were OP I'd redesign my program so that an object doesn't need to know what contains it (that makes no sense) but that's not my concern so I don't see any reason to delete my answer.

Simon Over a year ago

First, thank you for the answer. Second, is there a short way to explain what a circular reference is bad design? I was a computer science major briefly, but much of my coding is self-taught, and I don't think I ever learned why the DIP is so important. Thank you.

selplacei Over a year ago

@Simon one good reason is that circular references will never be garbage collected. But, on a bigger scale, it's good to structure objects in a way that represents data: when a container has a member object, the member represents some data that the container uses for its own purposes. It's usually a sign of bad engineering if containers and members need to know about each other: you could, for example, instead pass some sort of "global state" object around, or move some functionality to the container.

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