class Thread(db.Model):
  members = db.StringListProperty()

  def user_is_member(self, user):
    return str(user) in self.members

and

thread = Thread.get(db.Key.from_path('Thread', int(id)))
is_member = thread.user_is_member(user)

but the error is :

Traceback (most recent call last):
  File "D:\Program Files\Google\google_appengine\google\appengine\ext\webapp\__init__.py", line 511, in __call__
    handler.get(*groups)
  File "D:\Program Files\Google\google_appengine\google\appengine\ext\webapp\util.py", line 62, in check_login
    handler_method(self, *args)
  File "D:\zjm_code\forum_blog_gae\main.py", line 222, in get
    is_member = thread.user_is_member(user)
AttributeError: 'NoneType' object has no attribute 'user_is_member'

why ?

thanks