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> 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.