#15862: Mutability of tableaux, for the n-th time
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Reporter: darij | Owner:
Type: defect | Status: new
Priority: major | Milestone: sage-6.2
Component: | Keywords: tableaux, sage-combinat,
combinatorics | mutability
Merged in: | Authors:
Reviewers: | Report Upstream: N/A
Work issues: | Branch:
Commit: | Dependencies:
Stopgaps: |
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Tableaux in Sage are mutable objects, at least indirectly:
{{{
sage: T = Tableau([[1,2],[2]])
sage: t0 = T[0]
sage: t0[1] = 3
sage: T
[[1, 3], [2]]
}}}
This in itself is probably not a bug, although not the kind of behavior I
like either (what exactly is sped up by mutability of tableaux?). But
there are things which probably are bugs given this behavior:
1. Tableaux are hashed by reference:
{{{
sage: T = Tableau([[1,2],[2]])
sage: hash(T)
-7723024261707595164
sage: T[0][1] = 4
sage: hash(T)
-7723024261707595164
}}}
2. Line 311 of `sage/combinat/tableau.py` says:
{{{
# Since we are (suppose to be) immutable, we can share the
underlying data
}}}
But we are not immutable. This comment line is supposed to provide
justification for initializing the tableau as a `CombinatorialObject`, but
the docstring of `CombinatorialObject` says that "CombinatorialObjects are
shallowly immutable, and the intention is that they are semantically
immutable". The latter is not satisfied for tableaux.
If we want tableaux to be mutable, why wrap them inside such a class? If
we want them to be immutable, wouldn't it be right to encode them as
CombinatorialObjects of CombinatorialObjects? Or is the speed cost for
this too steep? And, finally, what is it that CombinatorialObject does
that tuple does not?
And, on a related note, does Sage provide a class for immutable
dictionaries? (I'm still hell-bent on implementing arbitrary-shaped
tableaux.)
3. Mutating tableaux poisons type judgments (or what passes for type
judgments in Sage):
{{{
sage: T = StandardTableau([[1,2],[3]])
sage: T[0][1] = 2
sage: isinstance(T, StandardTableau)
True
}}}
4. There is some action at a distance (although fortunately rare):
{{{
sage: T = SkewTableau([[1,2],[3]])
sage: S = T.to_tableau() # Tableau(S) doesn't work, wondering if it
should?
sage: S
[[1, 2], [3]]
sage: T
[[1, 2], [3]]
sage: T[0][1] = 5
sage: S
[[1, 5], [3]]
sage: T
[[1, 5], [3]]
}}}
5. How would I define Loday's Hopf algebra of tableaux if tableaux are
mutable?
--
Ticket URL: <http://trac.sagemath.org/ticket/15862>
Sage <http://www.sagemath.org>
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