On Fri, Oct 21, 2016 at 8:32 PM, Nick Coghlan wrote:

On 21 October 2016 at 17:09, Nathaniel Smith wrote:

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But that was 2.4. In the mean time, of course, PEP 442 fixed it so that finalizers and weakrefs mix just fine. In fact, weakref callbacks are now run *before* __del__ methods [2], so clearly it's now okay for arbitrary code to touch the objects during that phase of the GC -- at least in principle.

So what I'm wondering is, would anything terrible happen if we started passing still-live weakrefs into weakref callbacks, and then clearing them afterwards?

The weakref-before-__del__ ordering change in https://www.python.org/dev/peps/pep-0442/#disposal-of-cyclic-isolates only applies to cyclic garbage collection,so for normal refcount driven object cleanup in CPython, the __del__ still happens first:

>>> class C: ... def __del__(self): ... print("__del__ called") ... >>> c = C() >>> import weakref >>> def cb(): ... print("weakref callback called") ... >>> weakref.finalize(c, cb) >>> del c __del__ called weakref callback called

Ah, interesting! And in the old days this was of course the right way to do it, because until __del__ has completed it's possible that the object will get resurrected, and you don't want to clear the weakref until you're certain that it's dead. But PEP 442 already broke all that :-). Now weakref callbacks can happen before __del__, and they can happen on objects that are about to be resurrected. So if we wanted to pursue this then it seems like it would make sense to standardize on the following sequence for object teardown: 0) object becomes collectible (either refcount == 0 or it's part of a cyclic isolate) 1) weakref callbacks fire 2) weakrefs are cleared (unconditionally, so we keep the rule that any given weakref fires at most once, even if the object is resurrected) 3) if _PyGC_REFS_MASK_FINALIZED isn't set, __del__ fires, and then _PyGC_REFS_MASK_FINALIZED is set 4) check for resurrection 5) deallocate the object On further thought, this does still introduce one new edge case, which is that even if we keep the guarantee that no individual weakref can fire more than once, it's possible for *new* weakrefs to be registered after resurrection, so it becomes possible for an object to be resurrected multiple times. (Currently, resurrection can only happen once, because __del__ is disabled on resurrected objects and weakrefs can't resurrect at all.) I'm not actually sure that this is even a problem, but in any case it's easy to fix by making a rule that you can't take a weakref to an object whose _PyGC_REFS_MASK_FINALIZED flag is already set, plus adjust the teardown sequence to be: 0) object becomes collectible (either refcount == 0 or it's part of a cyclic isolate) 1) if _PyGC_REFS_MASK_FINALIZED is set, then go to step 7. Otherwise: 2) set _PyGC_REFS_MASK_FINALIZED 3) weakref callbacks fire 4) weakrefs are cleared (unconditionally) 5) __del__ fires 6) check for resurrection 7) deallocate the object There remains one obscure corner case where multiple resurrection is possible, because the resurrection-prevention flag doesn't exist on non-GC objects, so you'd still be able to take new weakrefs to those. But in that case __del__ can already do multiple resurrections, and some fellow named Nick Coghlan seemed to think that was okay back in 2013 [1], so probably it's not too bad ;-). [1] https://mail.python.org/pipermail/python-dev/2013-June/126850.html

This means the main problem with a strong reference being reachable from the weakref callback object remains: if the callback itself is reachable, then the original object is reachable, and you don't have a collectible cycle anymore.

>>> c = C() >>> def cb2(obj): ... print("weakref callback called with object reference") ... >>> weakref.finalize(c, cb2, c) >>> del c >>>

Changing that to support resurrecting the object so it can be passed into the callback without the callback itself holding a strong reference means losing the main "reasoning about software" benefit that weakref callbacks offer: they currently can't resurrect the object they relate to (since they never receive a strong reference to it), so it nominally doesn't matter if the interpreter calls them before or after that object has been entirely cleaned up.

I guess I'm missing the importance of this -- does the interpreter gain some particular benefit from having flexibility about when to fire weakref callbacks? Obviously it has to pick one in practice. (The async use case that got me thinking about this is, of course, exactly one where we would want a weakref callback to resurrect the object it refers to. Only once, though.) -n -- Nathaniel J. Smith -- https://vorpus.org

On Sat, Oct 22, 2016 at 3:01 AM, Nick Coghlan wrote:

On 22 October 2016 at 16:05, Nathaniel Smith wrote:

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On Fri, Oct 21, 2016 at 8:32 PM, Nick Coghlan wrote: But PEP 442 already broke all that :-). Now weakref callbacks can happen before __del__, and they can happen on objects that are about to be resurrected.

Right, but the resurrection can still only happen *in* __del__, so the interpreter doesn't need to deal with the case where it happens in a weakref callback instead - that's where the freedom to do the callbacks and the __del__ in either order comes from.

I think we're probably on the same page here, but to be clear, my point is that right now the resurrection logic seems to be (a) run some arbitrary Python code (__del__), (b) run a second check to see if a resurrection occurred (and the details of that check depend on whether the object is part of a cyclic isolate). Since these two phases are already decoupled from each other, it shouldn't cause any particular difficulty for the interpreter if we add weakref callbacks to the "run arbitrary code" phase. If we wanted to.

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There remains one obscure corner case where multiple resurrection is possible, because the resurrection-prevention flag doesn't exist on non-GC objects, so you'd still be able to take new weakrefs to those. But in that case __del__ can already do multiple resurrections, and some fellow named Nick Coghlan seemed to think that was okay back in 2013 [1], so probably it's not too bad ;-).

[1] https://mail.python.org/pipermail/python-dev/2013-June/126850.html

Right, that still doesn't bother me.

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Changing that to support resurrecting the object so it can be passed into the callback without the callback itself holding a strong reference means losing the main "reasoning about software" benefit that weakref callbacks offer: they currently can't resurrect the object they relate to (since they never receive a strong reference to it), so it nominally doesn't matter if the interpreter calls them before or after that object has been entirely cleaned up.

I guess I'm missing the importance of this -- does the interpreter gain some particular benefit from having flexibility about when to fire weakref callbacks? Obviously it has to pick one in practice.

Sorry, my attempted clarification of one practical implication made it look like I was defining the phrase I had in quotes. However, the "reasoning about software" benefit I see is "If you don't define __del__, you don't need to worry about object resurrection, as it's categorically impossible when only using weakref callbacks". Interpreter implementors are just one set of beneficiaries of that simplification - everyone writing weakref callbacks qualifies as well.

I do like invariants, but I'm having trouble seeing why this one is super valuable. I mean, if your object doesn't define __del__, then it's also impossible to distinguish between a weakref causing resurrection and a strong reference that prevents the object from being collected in the first place. And certainly it's harmless in the use case I have in mind, where normally the weakref would be created in the object's __init__ anyway :-).

However, if you're happy defining __del__ methods, then PEP 442 means you can already inject lazy cyclic cleanup that supports resurrection:

>>> class Target: ... pass ... >>> class Resurrector: ... def __init__(self, target): ... _self_ref = "_resurrector_{:d}".format(id(self)) ... self.target = target ... setattr(target, _self_ref, self) ... def __del__(self): ... globals()["resurrected"] = self.target ... >>> obj = Target() >>> Resurrector(obj) <__main__.Resurrector object at 0x7f42f8ae34e0> >>> del obj >>> resurrected Traceback (most recent call last): File "<stdin>", line 1, in <module> NameError: name 'resurrected' is not defined >>> import gc >>> gc.collect(); gc.collect(); gc.collect() 6 4 0 >>> resurrected <__main__.Target object at 0x7f42f8ae3438>

Given that, I don't see a lot of benefit in making weakref callbacks harder to reason about when __del__ + attribute injection already makes this possible.

That's a cute trick :-). But it does have one major downside compared to allowing weakref callbacks to access the object normally. With weakrefs you don't interfere with when the object is normally collected, and in particular for objects that aren't part of cycles, they're still collected promptly (on CPython). Here every object becomes part of a cycle, so objects that would otherwise be collected promptly won't be. (Remember that the reason I started thinking about this was that I was wondering if we could have a nice API for the asyncio event loop to "take over" the job of finalizing an object -- so ideally you'd want this finalizer to act as much like a regular __del__ method as possible.) Anyway I doubt we'll see any changes to this in the immediate future, but it's nice to get a sense of what the possible design landscape looks like... -n -- Nathaniel J. Smith -- https://vorpus.org