#10963: More functorial constructions
-------------------------------------+-------------------------------------
Reporter: nthiery | Owner: stumpc5
Type: enhancement | Status: needs_work
Priority: major | Milestone:
Component: categories | Resolution:
Keywords: | Merged in:
Authors: Nicolas M. Thiéry | Reviewers: Simon King
Report Upstream: N/A | Work issues: Reduce startup time
Branch: | by 5%. Avoid "recursion depth
Dependencies: #11224, #8327, | exceeded (ignored)".
#10193, #12895, #14516, #14722, | Commit:
#13589, #14471 | Stopgaps:
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Comment (by SimonKing):
`WeakValueDictionary` uses
{{{
#!python
def remove(wr, selfref=ref(self)):
self = selfref()
if self is not None:
del self.data[wr.key]
}}}
and `WeakKeyDictionary uses
{{{
def remove(k, selfref=ref(self)):
self = selfref()
if self is not None:
del self.data[k]
}}}
as a call-back.
And I think I see why `WeakValueDictionary` does not crash. Recall from
comment:95 that I did (of course with more layers)
{{{
M[b]=a
M[c]=b
M[d]=c
}}}
and the only elements with a strong reference being kept are d and a. When
deleting a, then successively the items keyed by b, c and d are removed
from the `WeakValueDictionary`.
But think for a moment what is happening during the callback, in the line
{{{
del self.data[wr.key]
}}}
When this is first called, `wr` is a weak reference pointing to a, and
`wr.key` is b. Hence, when `del self.data[wr.key]` is executed, then there
is still a ''strong'' reference to b, namely in wr.key. Only when the call
to `remove()` is finished, `wr` will be released, and in this moment the
last reference to b is gone. Hence, `del self.data[wr.key]` is called
again, but this time `wr` points to b and `wr.key` is a strong reference
to c.
'''Conclusion:''' During the deletion of the dictionary item (b,a), there
is a strong reference to the key b. Hence, the deletion of the item (c,b)
will only be started after deletion of the item (b,a) is completed. Hence,
no recursion.
But for a `WeakKeyDictionary` things are different. There,we have
(of course with more layers)
{{{
M[b]=a
M[c]=b
M[d]=c
}}}
and we only keep a reference to a and d. When we delete d, then the item
(d,c) will be removed. In the line
{{{
del self.data[k]
}}}
there is no strong reference to the value of `self.data[k]`. Hence, while
`self.data[k]` is deleted, it could be that the callback of a weak
reference pointing to this value is invoked.
And this analysis gives rise to a solution: In the `TripleDictEraser` and
`MonoDictEraser`, one should not simply do `del bucket[i:i+3]` or `del
bucket[i:i+7]`, but one should assign a temporary variable to
`bucket[i+2]` or `bucket[i+6]` that will keep the value alive until the
call to the eraser is completed, thus, avoiding the recursion.
I suggest to open a separate ticket for this issue.
--
Ticket URL: <http://trac.sagemath.org/ticket/10963#comment:98>
Sage <http://www.sagemath.org>
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