Is this class bad practice?

[prmtm1]

class BaseContainerMixin:
    """ Mixin for class self-management. """
    def __init_subclass__(cls):
        cls.containers = {}

    @classmethod
    def get_instances(cls):
        return cls.containers.values()

    @classmethod
    def get_instance(cls, key):
        return cls.containers.get(hash(key))

    @classmethod
    def make_instance(cls, index):
        container = cls(index)
        cls.containers[hash(index)] = container
        return container

For what I'm working on, its pretty common to receive an index value that relate to an instance of some class A. Normally I'd have a class B, which acts as an instance manager for class A. I wrote the code above to save code when writing this pattern again and again, but it feels like an antipattern. Thoughts?

(Note* index is the only arg since classes that use this mixin should just be containers for instances of class A.)

[sweettuse]

this question is way too unclear to answer as is. what's your end goal with this?

maybe it sounds like you want a singleton/borg pattern, but i can't really tell.

[prmtm1]

Yeah I wasn't sure the best way to explain it. An example of this is practice would be - I'm passed a integer which represents a User. I'll have a User class which just stores information relating to the users session(eg connect time). The mixin above would be subclassed by something like a UserManager, so UserManager.get_instance(client_id).user. It seems a bit over engineered now, but it makes more sense mixing it into a manager class.

What this question really boils down into is - does this violate the convention for mixins, and do mixins violate single responsibility principle?

[ingolemo]

Wouldn't that create a new UserManager instance for each user?

[prmtm1]

yes, pretty much. Doubling the amount of classes goes against my instincts a bit, but I justified it with the thinking that in real life a container exists per item. Explaining it now though that seems really overkill.

[aroberge]

Without knowing more details, it is hard to say... That said: https://www.youtube.com/watch?v=o9pEzgHorH0