#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:51 ncohen]:
> > I've never heard before that Python uses it.
>
> Oh. Then I guess we are the only ones to use this `._immutable` flag.
Ahem. Then I guess there are many things in Python that I've never heard
of...
> > But one question on copying needs to be addressed: I think in some
places in Sage, `copy(X)` returns `X` if `X` is immutable, but in other
places in Sage, `copy(X)` always returns a ''mutable'' copy of `X`,
regardless whether `X` is mutable or not. What would you prefer for
graphs?
> >
>
> Hmmm... Well, I guess it makes more sense to get with "copy" the same
kind of object that was given as input.
From my perspective, this behaviour would be fine. But I also see this:
{{{
sage: M = matrix([[1,2,3],[4,5,6]])
sage: M.set_immutable()
sage: M.is_immutable()
True
sage: m = copy(M)
sage: m.is_immutable()
False
}}}
There is one crucial difference, though: A mutable matrix can be made
immutable, by `M.set_immutable()`. But a mutable graph can (currently?)
not easily be made immutable, since mutability crucially relies on the
backend, which is not the case for matrices.
Anyway, I don't think that there is a commonly accepted standard in Sage.
Since we have no `G.set_immutable()` method for graphs, I guess it is
better to have
- `copy(G)` has the same backend as G. In particular, mutability is
copied. And if it is immutable, then we should have `G is copy(G)`.
- If one wants an immutable/mutable copy of a mutable/immutable graph G,
then it should be done either by `Graph(G,backend='...')` or
`G.copy(backend='...')` respectively with a to-be-implemented
`mutable=True/False` keyword.
--
Ticket URL: <http://trac.sagemath.org/ticket/15278#comment:52>
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