#7921: Categories for extension types via __getattr___
-------------------------------------+---------------------------------
Reporter: nthiery | Owner: nthiery
Type: enhancement | Status: closed
Priority: major | Milestone: sage-4.3.2
Component: categories | Resolution: fixed
Keywords: | Merged in: sage-4.3.2.alpha0
Authors: Nicolas M. Thiéry | Reviewers: Robert Bradshaw
Report Upstream: N/A | Work issues:
Branch: | Commit:
Dependencies: | Stopgaps:
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Comment (by nthiery):
Replying to [comment:15 saraedum]:
> I have a question regarding this old ticket. The following fails:
> {{{
> sage: a = ModularForms(11,4).an_element()
> sage: a._test_category()
> }}}
> The reason for this is that `a` is not an extension class so
`_test_category` expects that the type of `a` inherits from
`a.parent().category().element_class`. Now if I try to do such inheritance
> {{{
> class HeckeModuleElement(sage.modules.module_element.ModuleElement,
HeckeModules.element_class):
> }}}
> I get a
> {{{
> metaclass conflict: the metaclass of a derived class must be a (non-
strict) subclass of the metaclasses of all its base
> }}}
If it's not an extension type, it really should inherit
`a.parent().category().element_class`.
Except in complicated cases (e.g. parents whose elements can belong to
several classes), this is taken care of automatically if a category is
specified to
`Parent.__init__`; otherwise one needs to use manually
`_make_element_class`. See sage.categories.primer (better with #10963
applied) and the
documentation of Category for details.
Most likely the issue here is that the category is not specified
properly.
> Anyway, I'm getting the feeling that such inheritance does not happen
anywhere (partly because most element classes are extension classes).
Most of my bread and butter element classes are *not* extension
classes :-) Just for one example:
{{{
sage: x = CombinatorialFreeModule(QQ, [1,2,3]).an_element()
sage: x.__class__
<class
'sage.combinat.free_module.CombinatorialFreeModule_with_category.element_class'>
sage: x.__class__.__bases__
(sage.combinat.free_module.CombinatorialFreeModuleElement,
sage.categories.vector_spaces.VectorSpaces.WithBasis.element_class)
}}}
> So is it safe to remove this check from `_test_category`?
Please don't: so far failures here have always pointed to actual bugs.
Cheers,
Nicolas
--
Ticket URL: <http://trac.sagemath.org/ticket/7921#comment:16>
Sage <http://www.sagemath.org>
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