Create an incremental ID in a Python class with subclasses each maintaining their own ID series

Question

I want to create a class wherein each object of the class is given a unique sequential ID starting at 0. I also want each subclass to have sequential IDs starting at 0 as well, with numbering unaffected by parent or sibling classes.

Using the answer to a similar question, I've created the class Foo

class Foo:
    id_iter = itertools.count()
    def __init__(self):
        self.id = next(self.id_iter)

All instances of Foo will have a sequential id value. I also want to create a class Bar that inherits from Foo such that

>>> Foo().id
0
>>> Foo().id
1
>>> Foo().id
2
>>> Bar().id  # The first Bar id starts at 0 rather than 3
0

If I create Bar like:

class Bar(Foo):
    def __init__(self):
        super(Bar, self).__init__()

The above call to Bar().id would return 3 because both classes are using the same iterator. Alternatively, I could add id_iter = itertools.count() back into my Bar definition and get the output I'm expecting. However, I'm making multiple subclasses of Foo, and don't want to have to add that line to each class; I want all subclasses of Foo to automatically inherit the ID functionality without having to add anything extra to the subclass definition. How can I achieve this?

Answer 1

You can't do this with a class attribute as class attributes are not inherited. One method could be to use a separate dictionary for keeping count of classes. For example:

import itertools


class ClassCounter(object):
    counters = {}

    @classmethod
    def get_counter(cls, class_to_count):
        cls.counters.setdefault(class_to_count, itertools.count())
        return next(cls.counters[class_to_count])


class Foo:
    def __init__(self):
        self.id = ClassCounter.get_counter(self.__class__)


class Bar(Foo):
    def __init__(self):
        super(Bar, self).__init__()


if __name__ == '__main__':
    foo1 = Foo()
    foo2 = Foo()
    foo3 = Foo()
    bar1 = Bar()
    bar2 = Bar()
    print(f"{foo1.id=}, {foo2.id=}, {foo3.id=}, {bar1.id=}, {bar2.id=}")

This should print:

foo1.id=0, foo2.id=1, foo3.id=2, bar1.id=0, bar2.id=1