#13394: Write a WeakValueDictionary with safer key removal
-------------------------------------+-------------------------------------
Reporter: nbruin | Owner: rlm
Type: enhancement | Status: needs_review
Priority: major | Milestone: sage-5.13
Component: memleak | Resolution:
Keywords: | Merged in:
Authors: Simon King | Reviewers:
Report Upstream: None of the above | Work issues:
- read trac for reasoning. | Commit:
Branch: | fab0ed4112b9f798e2690f4c885b57cd711ea698
u/SimonKing/ticket/13394 | Stopgaps:
Dependencies: |
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Comment (by SimonKing):
Haha! Python's <dict> does in fact '''not''' prevent this from happening!
I think I have just found a bug in Python!
Here is an example that exposes the problem:
{{{
sage: import sage.misc.weak_dict
sage: D = sage.misc.weak_dict.WeakValueDictionary()
sage: class Key(object):
....: def __hash__(self):
....: print "hash of",id(self)
....: return 5
....: def __cmp__(self, other):
....: print "inserting 5"
....: D[5] = ZZ
....: return self is other
sage: D[Key()] = QQ
hash of 215823980
sage: D[Key()] = ZZ
hash of 216179948
inserting 5
Traceback (most recent call last):
...
RuntimeError: Can not add items while iterating over the dictionary
}}}
And this is what Python's dict does. There is no error, but it silently
fails:
{{{
sage: D = {}
sage: class Key(object):
....: def __hash__(self):
....: print "hash of",id(self)
....: return 5
....: def __cmp__(self, other):
....: print "inserting 5"
....: D[5] = ZZ
....: return self is other
sage: D[Key()] = QQ
hash of 215853068
sage: D[Key()] = ZZ
hash of 215853580
inserting 5
sage: len(D)
2
sage: D.keys()
[<__main__.Key at 0xcdda80c>, 5]
sage: id(_[0])
215853068
}}}
In other words, one of the second of the explicitly inserted items is
missing! That's a bug, I believe.
The same of course also happens with Python's weak value dictionaries (of
course, since they use a dict internally). Hence, Sage's
`CachedRepresentation` has been running into this bug all the time.
--
Ticket URL: <http://trac.sagemath.org/ticket/13394#comment:47>
Sage <http://www.sagemath.org>
Sage: Creating a Viable Open Source Alternative to Magma, Maple, Mathematica,
and MATLAB
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