How to debug "TypeError: Can't pickle objects in acquisition wrappers." in Plone - plone

Question

I have a handler that adds a Member to a Group. The last line in this handler causes an error:
TypeError: Can't pickle objects in acquisition wrappers.
> /home/mnieber/.buildout/eggs/ZODB3-3.10.3-py2.6-linux-i686.egg/ZODB/serialize.py(431)_dump()
430 self._p.dump(classmeta)
--> 431 self._p.dump(state)
432 self._file.truncate()
In the pdb debugger I can see that indeed Plone is trying to pickle a value that is an Acquisition wrapper:
ipdb> state
((((<PloneUser 'newuser#usecm.com'>, ('Default_Group',), 'maarten#usecm.com', ('PAS',)),),),)
ipdb> type(state[0][0][0][0])
<type 'Acquisition.ImplicitAcquisitionWrapper'>
However, I cannot see which object is being pickled, and therefore I have no idea which part of my code needs fixing. My question is: how should I go about debugging this error? I have tried looking at all the stack frames, but none of them reveal which object is being serialized.
The handler is this one (run_insecure is a decorator that I use to temporarily install a new security manager that avoids a NotAuthorized error when adding the new member):
#adapter(IPrincipalCreatedEvent)
#run_insecure
def userCreatedHandler(event):
portal_groups = getToolByName(getSite(), "portal_groups")
membersGroup = portal_groups.getGroupById('Default_Group')
membersGroup.addMember(event.principal)
The full error is this one:
Traceback (innermost last):
Module ZPublisher.Publish, line 134, in publish
Module Zope2.App.startup, line 301, in commit
Module transaction._manager, line 89, in commit
Module transaction._transaction, line 329, in commit
Module transaction._transaction, line 443, in _commitResources
Module ZODB.Connection, line 567, in commit
Module ZODB.Connection, line 623, in _commit
Module ZODB.Connection, line 658, in _store_objects
Module ZODB.serialize, line 422, in serialize
Module ZODB.serialize, line 431, in _dump
TypeError: Can't pickle objects in acquisition wrappers.
> /home/mnieber/.buildout/eggs/ZODB3-3.10.3-py2.6-linux-i686.egg/ZODB/serialize.py(431)_dump()
430 self._p.dump(classmeta)
--> 431 self._p.dump(state)
432 self._file.truncate()

Answer 1

I got this kind of problem with pickle, and solved by debugging like you did.
Pickle (used to store objects in ZODB) is trying to serialize your PloneUser, and raising this acquisition wrapper error.
In my case, I was wrapping the portal_workflow object into another class, and had to inherit it from pickle.Pickler, and override __getstate__ method to solve my problem.
This method is called by pickle in order to serialize your object. If you override this method, and return your object.__dict__ without this PloneUser, than this error would not be raised.
This question (although with not your exact problem) have more info about what I'm trying to say.
Nice you could solve your problem.