#15278: Hash and equality for graphs
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Reporter: SimonKing | Owner:
Type: defect | Status: needs_info
Priority: major | Milestone: sage-6.1
Component: graph theory | Resolution:
Keywords: | Merged in:
Authors: Simon King | Reviewers:
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/SimonKing/ticket/15278 | 2fc8a772ee12fce7ac6abc4ecf9916f4746f5ee2
Dependencies: #12601, #15491 | Stopgaps:
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Comment (by SimonKing):
Replying to [comment:39 ncohen]:
> HMmmmm... Don't you touch cachefunc.pyx in commit
126b03609e9bae78955ccbc97e36f1924a79603b ? That's when you merge #12601 in
the present branch.
Splitting hair...
> Gosh, I'm going to hate these git dependencies.
+1
> - In `.copy()`, you add a line before everything else to handle your
case. Couldn't you add a line after the second "if" block, the one after
which we know for sure that `data_structure` has been defined, saying "if
data_structure == "static_sparse" and hasattr(self,'_immutable')" then
return self ?
I am working on it.
> By the way, is an object considered mutable if it HAS a ._immutable
variable equal to False ?
Hmm. Would this be possible? I think a graph should be considered
immutable if and only if the static backend is used---or rather if and
only if ''some'' static backend is used: Perhaps there will be more static
backends in future.
So, instead of relying on an attribute `._immutable` that the user could
mess with, one could test for the type of the backend when we need to know
whether a graph is immutable.
Problem: If I am not mistaken, people currently ''do'' mess with the
attribute `._immutable`, if I am not mistaken.
Would it be feasible to try to remove the `._immutable` attribute (and
testing the type of the backend instead), see how much fails, and fix the
failing code by using "proper" immutable graphs?
> - Why wouldn't we add (later) a "immutable" flag to the constructor and
to "copy" ?
Would a flag in the "copy" function be supported by Python?
> It would be nice to call `Graph(graphs.PetersenGraph(),immutable=True)`
? `O_o`
Yes. Only problem: We would then need to decide what to do if
"immutable=False" and "backend="static_sparse" is simultaneously used.
> - What about an exception message for `_hash_` that would give the hint
that hashable graphs can be built ? Something like "This graph is mutable,
and thus not hashable. To make it mutable, read the doc of Graph.copy."
OK.
--
Ticket URL: <http://trac.sagemath.org/ticket/15278#comment:46>
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