Iterating through a resultset and adding news fields to the objects results in a corrupted iterator.

A simple test like the following code iterates many times over the same objects ... am I missing something? (see the attached file for a complete example)

1. 1. sample code ##
      total_obj = db.test.find().count()
      count = 0
      for item in db.test.find():
      db.test.update( {'_id':_id}

      , {'$set' : {'some_new_data' : random.randint(0, 1000)}})
      count = count +1

print count,total_obj

1. 1. /sample code ##

script usage:
python mongo_find_and_update.py test_db