[Python-Dev] Is there any remaining reason why weakref callbacks shouldn't be able to access the referenced object? (original) (raw)

Nathaniel Smith njs at pobox.com
Sat Oct 22 14:22:12 EDT 2016

On Sat, Oct 22, 2016 at 3:01 AM, Nick Coghlan <ncoghlan at gmail.com> wrote:

On 22 October 2016 at 16:05, Nathaniel Smith <njs at pobox.com> wrote:

On Fri, Oct 21, 2016 at 8:32 PM, Nick Coghlan <ncoghlan at gmail.com> 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.

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. 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): ... selfref = "resurrector{:d}".format(id(self)) ... self.target = target ... setattr(target, selfref, 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 "", line 1, in 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

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