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?
Best,
Alex
--
Alex Ghitza -- Lecturer in Mathematics -- The University of Melbourne
-- Australia -- http://www.ms.unimelb.edu.au/~aghitza/
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