If you've ever tried to use __slots__, you might have noticed that it comes with a pleasant side-effect: it makes it impossible to assign extra, garbage attributes to your objects. While normally not a serious problem, objects with extra, unintentional crud stuck to them can make object databases hell to work with as you struggle to figure out where it came from or why it is in your database but not in memory. Therefore, this semantic feature is handy in addition to reducing the memory footprint of objects if you have large numbers of them lying around.

If you've noticed this, you've also probably noticed that it is damn near impossible to use __slots__ because you have no control over what slots are used from your base classes.

Here is a solution to this problem that I have been working on for a while: SlotMachine. While it breaks isinstance - you weren't using isinstance anyway, right? - it does do the sane thing that you would expect a cooperative __slots__ implementation to do; you can subclass from random other classes (provided that you properly specify all their attributes as slots on your object) and other SlotMachines, and even other objects that define __slots__, without giving up the explicit specification of attributes or the ability to inherit from things.

I'll add some more comments to that file later tomorrow, I think.