On Fri, Apr 24, 2009 at 11:05 PM, Alex Ghitza <aghi...@gmail.com> wrote:
>
> Hi,
>
> I'm still working on fixing up schemes/generic/morphism.py, and I'm
> still having trouble with comparisons. I have written __cmp__()
> methods, but they don't seem to be called. Here's a concrete example.
> The definition of __cmp__ for SchemeMorphism_id is below:
>
> {{{
> class SchemeMorphism_id(SchemeMorphism):
> """
> The identity morphism from `X` to itself.
> """
>
> def __cmp__(self, other):
> if not isinstance(other, SchemeMorphism_id):
> return cmp(type(self), type(other))
>
> return cmp(self.parent(), other.parent())
> }}}
>
> Now if I do
>
> {{{
> sage: X = Spec(ZZ)
> sage: X.identity_morphism().parent() == X.identity_morphism().parent()
> True
> sage: X.identity_morphism() == X.identity_morphism()
> ---------------------------------------------------------------------------
> RuntimeError Traceback (most recent call last)
>
> /home/ghitza/.sage/temp/artin/25555/_home_ghitza__sage_init_sage_0.py
> in <module>()
>
> /opt/sage-devel/local/lib/python2.5/site-packages/sage/structure/element.so
> in sage.structure.element.Element.__richcmp__
> (sage/structure/element.c:6021)()
>
> /opt/sage-devel/local/lib/python2.5/site-packages/sage/structure/element.so
> in sage.structure.element.Element._richcmp
> (sage/structure/element.c:5385)()
>
> /opt/sage-devel/local/lib/python2.5/site-packages/sage/structure/coerce.so
> in sage.structure.coerce.CoercionModel_cache_maps.canonical_coercion
> (sage/structure/coerce.c:7059)()
>
> RuntimeError: BUG in map, returned None Scheme endomorphism of
> Spectrum of Integer Ring
> Defn: Identity map <type 'sage.categories.morphism.CallMorphism'>
> Call morphism:
> From: Set of points of Spectrum of Integer Ring defined over Integer Ring
> To: Set of points of Spectrum of Integer Ring defined over Integer Ring
> }}}
>
>
> So for some reason the correct __cmp__ is not called here. I've
> tracked it down to the following comment in structure/element.pyx:
>
> # For a *Python* class just define __cmp__ as always.
> # But note that when this gets called you can assume that
> # both inputs have identical parents.
> #
> # If your __cmp__ methods are not getting called, verify that the
> # canonical_coercion(x,y) is not throwing errors.
>
> The last part seems to be what's happening. But what does that mean,
> and how can I fix it?
Try calling canonical_coercion explicitly. What error is being thrown!?
William
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