> I don't think 2 is important.  Does the context of the call matter?
> It is merely a question of whether a finalizer will or will not do
> something trivial.

It would affect how I would write such functions.  If I knew that it wouldn't be called until the object was already garbage, then I be inclined to move parts of tp_del there, and break the cycle.

> Finalizers implemented in python can never run from garbage 
> collection.  the potential to do harm is simply too great.

__del__ methods do run, even if an object was collected by the cycle detector.  And they can't do any harm that couldn't also be done by a C finalizer.

The only change from today's situation with __del__ is that once an object is known to be cyclic garbage, it would get a chance to break the cycles itself (or, admittedly, to rescue itself) before the cycle-breaker began making arbitrary decisions or gave up and stuffed it in the uncollectable list.

Even in your case, instead of setting a timer to clean out garbage, you could have the garbage itself notify your cleaner that it needed attention.