The call to ensure-class without

:metaclass 'py-core-type

succeeds.

The call with the :metaclass argument fails inside change-class:

(defmethod change-class ((instance standard-object) (new-class standard-class)
&rest initargs)
(unless (class-finalized-p new-class)
(finalize-inheritance new-class))
(let ((cpl (class-precedence-list new-class)))
(dolist (class cpl)
(macrolet
((frob (class-name)
`(when (eq class (find-class ',class-name))
(error 'metaobject-initialization-violation
:format-control "~@<Cannot ~S objects into ~S
metaobjects.~@:>"
:format-arguments (list 'change-class ',class-name)
:references (list '(:amop :initialization ,class-name))))))
(frob class)
(frob generic-function)
(frob method)
(frob slot-definition))))
(change-class-internal instance new-class initargs))

CLASS is on the class precedence list of PY-CORE-TYPE.

Thanks, I'm going to look into this.

- Willem