Welcome to the common RustPython development rabbit hole. Well, I only have general debug advices. It can be either a bug here or combination of other incompatabilty issues there - because getset is shown in error message, I suspect our incomplete getset_descriptor implementation may be releated.
General idea is breaking it down enough and see how it is going. When an exception is raised during initialization(VirtualMachine::initialize) step, you will want to set breakpoint and add some print_exception or other debugging helpers in VirtualMachine::initialize function.

I think the trial was really good.

I am currently on vacation. I can try a quick look, but not able to look deep down stuff for a while. (To be honest, usually not very helpful for these kind of problems even if I do that)

Alright, I'll keep digging deeper. Have a nice vacation!

@dannasman Do you have a specific goal to achieve? Otherwise looking around similar issues may be helpful. I also have many abandoned patches when I got stucked https://github.com/RustPython/RustPython/pulls/youknowone

I was searching for places where PyObject_TypeCheck should be used in the code and found the _check() function. The original goal was to add type_check into it just like in CPython. Further investigation showed that the implementations between RustPython and CPython have some differences when it comes to descriptors. To my current knowledge In RustPython generic_getattr_opt() function descriptor objects seem to always get a type getset_descriptor whereas in corresponding CPython function _PyObject_GenericGetAttrWithDict the descriptor objects are assigned the same type (or subtype) as the self object (and this is later checked with PyObject_TypeCheck to make sure this is the case). So it might be the case that _check() does not actually need type_check() since the type of descr does not depend on the type of zelf.