I'm using mango.session, witch stores django sessions in mongodb.
Storing sessions works ok.
But when i'm trying to restore session it comes empty.
When debugging into deeps, found that pickle.loads(pickled) - session_data throws exception UnicodeEncodeError:

Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\pymongo-2.2-py2.7-win32.egg\bson\objectid.py", line 223, in __setstate__
    self.__id = value.encode('latin-1')

UnicodeEncodeError: 'latin-1' codec can't encode characters in position 3-4: ordinal not in range(256)

Then i tried to eliminate error and added to except clause UnicodeEncodeError (to existing UnicodeDecodeError and AttributeError).
Works like a charm.

Then I tried to reproduce this behavior in IDLE and had no luck. Only see UnicodeDecodeError with same data as I get from db.
Pickled string looked like:

'\x80\x02}q\x01(U\x12_auth_user_backendq\x02U\x12mango.auth.Backendq\x03U\r_auth_user_idq\x04cbson.objectid\nObjectId\nq\x05)\x81q\x06U\x0cO\xad\xb5\xf7\xbcu\x9e\x10p\x00\x00\x00q\x07bu.'

Here truncated code from django.contrib.sessions.backends.base.py SessionBase.load:

from django.utils.encoding import force_unicode
import base64
import pickle #django imports cPickle, both work same way
encoded_data = base64.decodestring(force_unicode('MWYxZWIxYWI3YTg3M2YyNTlmZmQxOWI3NTIzMDRmZjlkYTU5ZDBjYjqAAn1xAShVEl9hdXRoX3Vz\n\
ZXJfYmFja2VuZHECVRJtYW5nby5hdXRoLkJhY2tlbmRxA1UNX2F1dGhfdXNlcl9pZHEEY2Jzb24u\n\
b2JqZWN0aWQKT2JqZWN0SWQKcQUpgXEGVQxPrbX3vHWeEHAAAABxB2J1Lg==')) #this string came from mongodb sessions collection as session_data
hash, pickled = encoded_data.split(':', 1) #salt checks pass behind the scene
pickle.loads(pickled)

IDLE with this code works ok (UnicodeDecodeError supressed) and result dict as expected:

{'_auth_user_id': ObjectId('4fadb5f7bc759e1070000000'), '_auth_user_backend': 'mango.auth.Backend'}

But when this run in django app i see that int _setstate__ as value passed some binary value (see screenshot from eclipse debug variables) witch throws UnicodeEncodeError when trying to convert it to latin-1. And self.__id = value in except block (if I add UnicodeEncodeError to except) works right.

I'm a newbie to python and mongodb, maybe that is my mistake. Thanx.

See also: https://github.com/mongodb/mongo-python-driver/commit/7474f5cde80fb0bd0e4a1bf4cbb88b4902131810#commitcomment-1354855