The issue stems from this:
sage: Poset(b)
Finite lattice containing 1 elements
sage: a == b
False
The first behavior is a feature, as b already knows it is a lattice, so b
should act be an idempotent wrt to Poset. IMO, the equality a == b should
be true as knowing you're a lattice is just extra information, but not part
of the structure. However, it is a drawback of posets being unique
representations as we don't want to check for being a lattice when passing
data. So I'm thinking what the lattice code should do is create the poset
object, and then add the lattice classes to the MRO of the returned object.
AFAIK, this should play nicely with the unique representation behavior and
fix the issue at hand. The alternative is instead of checking for
subclasses, check for exact class equality in Poset. We might also want to
consider moving over to a UniqueFactory.
Best,
Travis
On Wednesday, September 30, 2015 at 5:23:12 AM UTC-5, Jori Mäntysalo wrote:
>
> Is this a feature or a bug?
>
> a = Poset({1:[]})
> b = LatticePoset({1:[]})
> c1 = Poset(b)
> c2 = Poset(b.hasse_diagram())
> a==c1, a==c2
>
> outputs False, True. But this is quite a surprise for a user.
>
> --
> Jori Mäntysalo
>
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