Python uses cycle breaking i.e. it chooses an object on a circular path and simply deletes it, which essentially bricks the entire connected set for reference counting purposes. This means that if you ran __del__ code in this circumstance it could access references to deleted objects, i.e. use after free.
> Python doesn't know the order in which it's safe to destroy objects that hold circular references to each other, so as a design decision, it just doesn't call the destructors for such methods!
