On Jul 17, 4:51 pm, Carl Witty <[EMAIL PROTECTED]> wrote:
> On Jul 17, 3:30 pm, John H Palmieri <[EMAIL PROTECTED]> wrote:
>
>
>
> > I have several different vector space bases for an algebra A, and I
> > have implemented this by having a class MyAlgebra, instances of which
> > have an attribute _basis_name which is a string naming the basis. I
> > have __cmp__ defined so that the basis is ignored: any two instances
> > are canonically isomorphic, and there is good coercion between them.
>
> > Now, the attribute _basis_name just affects the _repr_ and _latex_
> > methods for elements of the algebra; it has no other effect. My
> > question is this: when I multiply elements from algebras with two
> > different bases, I can't figure out how the parent of the result is
> > determined. Some documentation suggests that the parent should be
> > determined by the left factor, but that is not consistently what's
> > happening.
>
> > I think I would like to force the result to have the same parent as
> > the left-hand factor, but by the time elements get to the _mul_
> > method, their parents have been changed (via coercion), so I can't
> > find out what that parent is. (Perhaps something is getting cached in
> > the coercion process, so things are getting coerced to the cached
> > algebra, not the left-hand parent?) What should I do? Does it sound
> > like I'm doing something wrong, and if so, what should I investigate?
>
> Currently it's built-in to coercion that if two parents are equal,
> then it doesn't matter which parent it picks. (And the decision is
> baked into various caches, etc., so the decision can depend on the
> previous history of commands in this session.)
>
> I've raised this issue as a problem a few weeks ago
> (seehttp://groups.google.com/group/sage-devel/browse_thread/thread/4520ed...),
> but nothing came of it yet.
>
> Even if this gets fixed, I don't think you can fix it so that coercion
> uses the parent of the left-hand value by changing your code; this
> would require changes inside the base coercion code.
Okay, thanks. Here's something from Section 2.8 of the Sage
Programming Guide:
ARITHMETIC __add__, __mul__, ...:: When doing a binary operation, if
the parents are not identical (in the sense of is ), determine if
precisely one _coerce_ map is defined; if so, apply it and do the
arithmetic operation. If both are defined, the parents are canonically
isomorphic, so use the left one. If neither are defined, raise a
TypeError. (Whether or not there is a coerce map between objects
should be cached for efficiency.)
I guess you're saying that this is wrong (the part that says "so use
the left one").
> Carl
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