Python – Google app engine and python and objects that pass by value

This is really a python language question, but its wrapped around a Google appengine specific problem.

We have

class User( db.Model ) :
  email = db.StringProperty()
  name = db.StringProperty()
  password = db.StringProperty()
  # more fields..

Because the user account is accessed so often, we keep a copy in session using gaeutilities (as a bonus question, is this bad on GAE? I thought I'd relieve the db a bit.)

class UpdateProfile( webapp.RequestHandler ):
  def post( self ):
    # User posting update to his name
    self.session = sessions.Session()

    #######
    # way#1: update the copy of the User object in SESSION, then .put() it
    self.session[ 'current_user' ].name = self.request.get( 'name' )
    self.session[ 'current_user' ].put()
    # does not work.
    #######

    #######
    # way#2: make a copy of the User object in SESSION, then .put() it
    user = self.session[ 'current_user' ]
    user.name = self.request.get( 'name' )
    user.put()
    # works to update the datastore, but the copy of user in self.session
    # is NOT UPDATED!  I thought Python was
    # pass-by-reference.  It is when you work with lists anyway.
    # Why isn't it "acting right" with this type of object??
    #######


    #######
    # way#3: way that works.
    user = self.session[ 'current_user' ]
    user.name = self.request.get( 'name' )
    user.put()
    self.session[ 'current_user' ] = user
    # works completely
    #######

What is happening in each of these 3 cases? Why don't cases 1 and 2 work?