Sorry, the previous message was written via a phone because of blackout, so the formatting is a tine bit messy.

__slotnames__ is set in copyreg._slotnames(). If you want to keep a list of all slots you should save it in __slotnames__, either using copyreg._slotnames() directly or by duplicating its code.

But as for the original issue, I do not think that the current behavior is incorrect. At least it is consistent with the behavior of regular classes.