class Recipe(db.Model):
  text = db.TextProperty()
  owner = db.UserProperty()
  def put(self):
    if not self.owner:
      self.owner = users.get_current_user()
    return db.Model.put(self)
  def get_by_key_name(cls, key_name, parent=None):
    entity = super(Recipe, cls).get_by_key_name(key_name, parent)
    if entity.owner != users.get_current_user():
      raise Exception("Access denied")
    return entity
# Set the owner automatically
r = Recipe(key_name="secret sauce")
r.text = "ketchup and mayonnaise"
r.put()
# Only allow the owner to get the record
r = Recipe.get_by_key_name("secret sauce")

====================================================================

See http://fi.am/entry/urlsafe-base64-encodingdecoding-in-two-lines/

You can do this using Python's built-in base64 library in just two lines of code:

import base64

def uri_b64encode(s):
     return base64.urlsafe_b64encode(s).strip('=')

def uri_b64decode(s):
     return base64.urlsafe_b64decode(s + '=' * (len(s) % 4))