Multiple sub classes inheriting one instance of their super class

Question

Suppose I have the following example:

import uuid

class A:
    def __init__(self):
        self.id = uuid.uuid4().hex

class B(A):
    def __init__(self):
        super().__init__()
        self.print_id()

    def print_id(self):
        print(self.id)

class C(A):
    def __init__(self):
        super().__init__()
        self.print_id()

    def print_id(self):
        print(self.id)

We have class B and C that are inheriting A. Once B or C are instantiated, they will print the unique ID created in the super class. The issue I am running into here is that each time B or C is instantiated, there is a brand new id created for class A. I would like to print/use the SAME ID for both class B and C. In other words, I only want to have one instance of class A to be used for B and C. Without involving any sort of logic surrounding singletons, how can I achieve this? How can class C know that class B has already initialized class A, therefore an ID already exists? I am attempting to create a base class that is initialized only once, and that ID is inherited by all the sub classes. Any help here would be appreciated.

You cannot inherit from instances, but from classes. The way your class is written, a new id gets created for every instance. — juanpa.arrivillaga, Mar 17 '20 at 17:50
@juanpa.arrivillaga thanks for this note. I have noticed this. If I would like to derive from a base that only creates this ID once, how can I achieve this? — Karanvir Ghumaan, Mar 17 '20 at 17:51
You essentially want each class to have a unique id, where every instance of those classes shares the same id? I'm still not exactly sure what you want. — juanpa.arrivillaga, Mar 17 '20 at 17:54

score 1 · Accepted Answer · edited Mar 17 '20 at 18:02

1

You can use a class attribute

e.g.

class A:
    static_id = uuid.uuid4().hex

    def __init__(self):
        pass

The rest of the code will be the same.

I changed the name of the id member, because it shadowed the name of a built-in function.

(This solution smells to me a little, but I do not understand your problem enough to suggest anything better).

edited Mar 17 '20 at 18:02

chepner

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answered Mar 17 '20 at 17:54

jon

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There's nothing static about a class attribute, in the sense that C/C++ use the term. – chepner Mar 17 '20 at 18:03
@chepner can you elaborate? – juanpa.arrivillaga Mar 17 '20 at 18:10
I would defer to the explanations of `static` given in answers to [this question](https://stackoverflow.com/q/572547/1126841). – chepner Mar 17 '20 at 18:15

Multiple sub classes inheriting one instance of their super class

1 Answers1