zope.copy.copy() copies __parent__ before discarding it

zope.copy.copy() does

    new = clone(obj)
    new.__parent__ = None

in that order, which means __parent__ and everything reachable from it (which is more or less the entire ZODB, if your objects have __parent__ references) are first copied and then discarded.

That assumes those objects can be copied, which is not always the case:
https://mail.zope.org/pipermail/zope-dev/2010-December/042202.html

How about making pickler.persistent_id in zope.copy.clone() check the object against a blacklist before passing it on to persistent.id? The blacklist could be an argument to clone(), and copy() would pass obj.__parent__ to it.