Shared class int across subclasses

Question

I want to have a counter which increments every time a subclass is instantiated. How would I achieve this such that the last statement below evaluates to True:

class Abstract(ABC):
    counter = 0

class A(Abstract):
    pass
class B(Abstract):
    pass

a = A()
b = B()
a.counter += 1
b.counter == 1

Currently each subclass gets its own counter, rather than sharing the one outlined in the superclass.

*"each subclass gets its own counter "* - no, `a.counter += 1` is not updating the `A` `counter` *or* the `Abstract` `counter`. Assigning to instance attributes assigns them *on the instance*. See e.g. https://stackoverflow.com/q/68645/3001761. — jonrsharpe, Oct 27 '20 at 10:18
This is True, in reality my code was calling cls.counter += 1 however. — Alesi Rowland, Oct 27 '20 at 12:50

score 2 · Accepted Answer · answered Oct 26 '20 at 11:45

2

Would this work for you?

global_counter = 0


class Abstract:
    def __init__(self):
        global global_counter
        global_counter += 1


class A(Abstract):
    def __init__(self):
        super().__init__()


class B(Abstract):
    def __init__(self):
        super().__init__()


a = A()
b = B()

print(global_counter)    # (output: 2)

answered Oct 26 '20 at 11:45

nihilok

1,325
8
12

It would work however I tend to avoid using global (probably due to articles I've read on avoiding it) – Alesi Rowland Oct 26 '20 at 12:03
Entirely depends how you use em. The trouble is every time you instantiate the class with the counter you instantiate the counter as 0 – nihilok Oct 26 '20 at 12:24
Yep, for some reason i thought subclasses share the same instance of super class attributes. Turns out they do but only for not base types I think. Building a "Counter" class which is effectively a fancy integer also works! – Alesi Rowland Oct 27 '20 at 10:10

score 0 · Answer 2 · answered Oct 27 '20 at 13:04

So I've implemented several different ways to achieve what I wanted:

Using a global keyword as suggested by @nihilok
Creating a custom Counter class to handle the integer value (essentially a fancy int).
using an int type in the super class and having its methods directly reference it using the super class's name instead of simply using the cls passed in during a class method.

My favourite (least amount of extra parts and most canonical to how I tend to write my objects) was the last method. Where the above translates to something like:

lass Abstract(ABC):
    counter = 0
    @staticmethod
    def increment():
        Abstract.counter += 1. # instead of cls.counter += 1

class A(Abstract):
    pass
class B(Abstract):
    pass

a = A()
b = B()
a.increment()
b.increment()
a.counter == b.counter # now true.

nice, although doesn't do what you set out in your question which is count on instantiations — nihilok, Oct 27 '20 at 13:09
True - sorry I was writing this up at work and just wrote up the main crux of the issue as was in a rush. to have it increment I believe you would just add __new__ to abstract and call the static method within that. — Alesi Rowland, Oct 28 '20 at 10:24

Shared class int across subclasses

2 Answers2