Hi!
Let P be a parent and P.Element=EC a subclass of Element.
- If EC is a Python class, then P.element_class is a dynamic class
obtained from EC and from P.category().element_class.
- If EC is a Cython class, then P.element_class is just EC. Why?
It should still inherit methods from P.category().element_class,
but (at least according to what I see at #23707) this is not the
case for __getitem__ (and I guess for other magical methods, too).
- Back to the case that EC is Python. Then, let's consider how
D = P.element_class (which is a dynamic class!) is pickled. Currently,
it is pickled as a dynamic class with the bases EC and
P.category().element_class.
I believe this is a bug. Namely, if in a newer version of Sage you
change EC to a Cython class (which I am doing in #23707), unpickling
an old pickle would still give you the old dynamic class, which in
the new version is *not* the same as P.element_class.
What I believe should be done:
- Use a dynamic class for P.element_class even if EC is Cython (unless
there is a compelling reason not to do so, but then please solve the
inheritance problem for the magical methods provided by the category
framework)
and/or
- Use the fact that when creating a dynamic class, one can provide
information on how it should be pickled. Namely, P.element_class should
be pickled as
getattr, (P, 'element_class')
so that in future it will be guaranteed that when unpickling an old pickle,
the correct element class would result.
Thoughts?
Best regards,
Simon
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