object_session can sometimes generate TypeError (patch included)

[Former user]

sqlalchelmy.orm.session.object_session function can sometimes generate TypeError exceptions, when session is garbage collected while some objects that were in that session exist (have _sa_session_id set) and there is lazy attribute load attempt on their attributes:

          File "c:\svn\sqlalchemy\lib\sqlalchemy\orm\attributes.py", line 44, in __get__
            return self.impl.get(instance._state)
          File "c:\svn\sqlalchemy\lib\sqlalchemy\orm\attributes.py", line 223, in get
            value = callable_()
          File "c:\svn\sqlalchemy\lib\sqlalchemy\orm\strategies.py", line 436, in __call__
            session = sessionlib.object_session(instance)
          File "c:\svn\sqlalchemy\lib\sqlalchemy\orm\session.py", line 1144, in object_session
            if instance in sess:
        exceptions.TypeError: argument of type 'NoneType' is not iterable

Attached simple patch adds "sess is not None" do this function, fixing the problem (in this particular case, result being sqlalchemy.exceptions.InvalidRequestError raised correctly).

[Mike Bayer]

thank you, 6d486fb2e77e338ac4d45ff5ed45561abb5c81b3.