#15692: Value of cached methods should not always be pickled
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Reporter: SimonKing | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.1
Component: pickling | Keywords: pickling hash cache
Merged in: | Authors:
Reviewers: | Report Upstream: N/A
Work issues: | Branch:
Commit: | Dependencies:
Stopgaps: |
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In #15278, the hash of graphs was turned into a `@cached_method`. If a
cached method is pickled, then the cached value is pickled as well. In the
case of a hash value, this is not always what we want, since the hash
value might depend on the memory address of an object, or worse: The hash
function might change in future implementations.
That's to say: If you save object O now, together with hash value 123, and
later load it, then hash(O) will still be 123, even if a newly constructed
object P with O==P might have hash(P)==456, because of a change in the
hash function.
This obviously is a problem. There is something called
`sage.misc.cachefunc.ClearCacheOnPickle`, but I don't know if this would
really help in this case. Also, this would clear ''all'' cache values at
once. Also, I don't know if it works for cached methods that do not take
arguments (beside `self`).
I suggest that we should instead have an optional parameter for
`sage.misc.cachefunc.CachedMethodCallerNoArgs` that determines whether or
not the cached value is preserved, and use it on graphs (and future
applications of cached method on `__hash__`).
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Ticket URL: <http://trac.sagemath.org/ticket/15692>
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