Hi!
On Fri, Nov 08, 2013 at 09:47:08AM +0000, Simon King wrote:
> On 2013-11-08, Vincent Delecroix <20100.delecr...@gmail.com> wrote:
> > I was working with #15378 and I wondered why SchemeMorphism inherits
> > from Element and not from Morphism as they should.
>
> It *should* in fact inherit from both. In fact, some time ago, I tried
> to provide both Morphism and Element as base classes for SchemeMorphism.
>
> The problem is: Python allows you to use both base classes (it believes
> that both base classes have a compatible layout), but then some cdef
> attribute of Element gets confused with some cdef attribute of Morphism.
>
> Hence, in fact these two base classes do *not* have compatible layout,
> and it is a bug that Python does not raise an error when you try to
> create a class that uses both base classes at the same time.
I am a bit confused, since Morphism already inherits from Element:
sage: from sage.categories.morphism import Morphism
sage: Morphism.mro()
[sage.categories.morphism.Morphism, sage.categories.map.Map,
sage.structure.element.Element, ...]
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.
> So, in this case, multiple inheritance is a no-op, and thus the
> category framework is your friend...
Well, up to one detail: the category framework somewhat assumes that
parents inherit from Parent, elements from Element, and morphisms from
Morphism. Namely I guess in theory you could construct a parent not
inheriting from Parent, but it would have to duplicate the logic for
setting the class properly to inherit from categories, and there might
be some minor glitches elsewhere where some code does sanity checks
like:
assert isinstance(p, Parent)
I guess that, to move forward, we should implement two classes:
class ElementAndParent(...):
(and maybe variants like ParentAndMorphism, CategoryAndParent ...)
I guess it's all fine if those classes are Python classes, at least
for now; that's more for more advanced objects where pure speed is not
as critical as for basic arithmetic elements.
Then we have two options:
- 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.
Cheers,
Nicolas
--
Nicolas M. Thiéry "Isil" <nthi...@users.sf.net>
http://Nicolas.Thiery.name/
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