#14159: Don't install callbacks on values of TripleDict, MonoDict
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Reporter: nbruin | Owner: tbd
Type: PLEASE CHANGE | Status: new
Priority: major | Milestone: sage-5.8
Component: PLEASE CHANGE | Resolution:
Keywords: | Work issues:
Report Upstream: N/A | Reviewers:
Authors: | Merged in:
Dependencies: | Stopgaps:
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Comment (by SimonKing):
Replying to [comment:7 nbruin]:
> You have to make sure that the weakref object lives to see the death of
v (and ensure that you make such a weakref in the first place!)
Apparently I had wrong memories about my code, then. I had in mind that
the `TripleDict` has a weak reference with callback to the value.
> {{{
> sage: from sage.structure.coerce_dict import TripleDict
> sage: import gc
> sage: import weakref
> sage: class Foo: pass
> sage:
> sage: a = 1
> sage: b = 2
> sage: c = 3
> sage: v = Foo()
> sage: w = Foo()
> sage: T = TripleDict(13)
> sage: T[a,b,c] = weakref.KeyedRef(v,T.eraser,(id(a),id(b),id(c)))
> sage: h=T[a,b,c]
> sage: T[a,b,c] = w
> sage: del v
> sage: _ = gc.collect()
> sage: len([x for x in gc.get_objects() if isinstance(x,Foo)])
> 1
> sage: T[a,b,c] == w
> KeyError: (1, 2, 3)
> }}}
> I admit, the weakref surviving outside the dict is not a terribly likely
event, but from your code at #11521 it wasn't entirely clear to me that
you can absolutely rule it out.
Hm. This particular `TripleDict` is only used by hom, and hom doe not
override an existing entry. So, this particular case should be safe.
However, it still seems reasonable to me to let the callback check whether
the to-be-deleted value did not change.
--
Ticket URL: <http://trac.sagemath.org/sage_trac/ticket/14159#comment:8>
Sage <http://www.sagemath.org>
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