Hey everyone,
Currently we have the following behavior in Sage. This is okay:
sage: SGA4 = SymmetricGroupAlgebra(QQ, 4)
sage: SGA3 = SymmetricGroupAlgebra(QQ, 3)
sage: lift = SGA4.module_morphism(lambda x: SGA3.zero(), codomain=SGA3)
sage: SGA3.register_coercion(lift)
Now startup a new session:
sage: SGA4 = SymmetricGroupAlgebra(QQ, 4)
sage: SGA3 = SymmetricGroupAlgebra(QQ, 3)
sage: SGA3.has_coerce_map_from(SGA4)
False
sage: lift = SGA4.module_morphism(lambda x: SGA3.zero(), codomain=SGA3)
sage: SGA3.register_coercion(lift)
---------------------------------------------------------------------------
AssertionError Traceback (most recent call last)
<ipython-input-6-d757c9181cb9> in <module>()
----> 1 SGA3.register_coercion(lift)
/home/travis/sage/local/lib/python2.7/site-packages/sage/structure/parent.so
in sage.structure.parent.Parent.register_coercion (build/cythonized/sage/
structure/parent.c:13361)()
/home/travis/sage/local/lib/python2.7/site-packages/sage/structure/parent.so
in sage.structure.parent.Parent.register_coercion (build/cythonized/sage/
structure/parent.c:13271)()
AssertionError: coercion from Symmetric group algebra of order 4 over
Rational Field to Symmetric group algebra of order 3 over Rational Field
already registered or discovered
Darij and I have been bitten by this before, but we (at least I) thought
this was the coercion model being extra cautious. However I'm working with
Miguel Marco and Julian Rueth where Miguel's doing an implementation which
has a subclass D of another class C and is trying to setup and coercion
from an instance of D to C and it fails with the same method. We've
narrowed it to _coerce_from_hash is not behaving as it should (in both
cases, the hashes of the parents are different and == is false), and we've
narrowed it down to lines 2351-2 in parent.pyx:
if (mor is not None) or _may_cache_none(self, S, "coerce"):
self._coerce_from_hash.set(S,mor)
in that the morphism is None in this case. So what is the proper solution
for this? Is there some way we can work around this?
Thanks,
Travis
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