Hi!
Currently, I try to make homsets and morphisms compliant with what one
would usually do when implementing parents and elements---in particular,
the default __mul__ and __add__ should do something meaningful and should
thus not be overridden. In particular, one would implement addition in
subclasses by providing a method _add_, not __add__.
But this implies a subtle problem, in
sage.homology.chain_complex_morphism.ChainComplexMorphism.
I guess one would like to define it as follows,
class ChainComplexMorphism(Morphism, ModuleElement)
because it is a morphism, on the one hand, but a module element, on the
other hand.
Both Morphism and ModuleElement are extension classes with their own cdef
attributes/methods, but it *is* possible to inherit from both (Python
does not throw an error, i.e., it does not complain about incompatible
layouts).
If one makes ChainComplexMorphism use the __add__ method inherited from
ModuleElement, then eventually the line
return (<ModuleElement>left)._add_(<ModuleElement>right)
is executed. But "(<ModuleElement>left)._add_" returns a totally
different cdef method, namely "(<Map>left)._update_slots"! Example:
sage: cython("""
....: from sage.categories.map cimport Map
....: from sage.structure.element cimport ModuleElement
....: def test(a):
....: return <void*>((<ModuleElement>a)._add_) == <void
*>((<Map>a)._update_slots)
....: """)
sage: from sage.categories.map import Map
sage: class C(Map, ModuleElement): pass
....:
sage: test(a)
True
sage: a._add_(a)
Traceback (most recent call last):
...
... in sage.structure.element.ModuleElement._add_
(sage/structure/element.c:9801)()
... in sage.categories.map.Map._update_slots (sage/categories/map.c:2633)()
TypeError: 'C' object has no attribute '__getitem__'
So, _add_ can not be accessed, and in particular one can not use the
default __add__ (which is essential for using the coercion model) on a
class that inherits from both Map and ModuleElement.
Do you see a work around?
I asked this question on the cython-users list, too (sorry for those who
read both), but I think it is as much a question about sage than a
question about cython.
Best regards,
Simon
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