Python metaclasses

meta.py

class AutoIncrementIDMeta(type):
    __counter = {}

    def __call__(cls, *args, **kwargs):
        if cls.__name__ not in cls.__counter:
            cls.__counter[cls.__name__] = 0
        cls.__counter[cls.__name__] += 1
        instance = super().__call__(*args, **kwargs)
        instance.id = cls.__counter[cls.__name__]
        return instance


class AutoObjectSetMeta(type):

    def __new__(mcs, *args, **kwargs):
        cls = super().__new__(mcs, *args, **kwargs)
        cls.instances = set()
        return cls

    def __call__(cls, *args, **kwargs):
        instance = super().__call__(*args, **kwargs)
        cls.instances.add(instance)
        return instance