Hi Nicolas,
On 2013-11-10, Nicolas M. Thiery <nicolas.thi...@u-psud.fr> wrote:
> I am a bit confused, since Morphism already inherits from Element:
Yes, sorry, I meant to say: "ModuleElement".
> Anyway, I am going to widen a bit the scope of the discussion. On a
> similar note, we have a long waiting need for implementing parents
> that are also elements. No longer than Wednesday, that was requested
> again by Eric during his description of his sage-manifold library. So
> this might be a good time to think about how to implement this
> feature.
In one way this would be easier than a parent that at the same time is a
category. Namely, parents and categories have features that are called
the same but have a totally different meaning, such as:
Parent.element_class versus Category.element_class. For Parent versus
Element, I think different features are called differently.
However, Python knows that the base classes are incompatible:
sage: from sage.structure.element import Element
sage: from sage.structure.parent import Parent
sage: class C(Parent, Element): pass
Traceback (most recent call last):
...
TypeError: Error when calling the metaclass bases
multiple bases have instance lay-out conflict
> I guess that, to move forward, we should implement two classes:
>
> class ElementAndParent(...):
>
> (and maybe variants like ParentAndMorphism, CategoryAndParent ...)
Modulo lay-out conflicts...
> - Either there is a way to make ElementAndParent inherit both from
> Parent and Element by changing the layout of the base classes to be
> compatible (this sounds unlikely, but I am not an expert).
>
> - Or we should fix the category framework so that everything that
> quacks like an Element is accepted as an Element, and hopefully with
> minimal duplication of the inheritance-from-category handling.
The latter sounds easier (but slower) to me.
Best regards,
Simon
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