[Zope] Get userobject from python

Hello, I am trying to find a way to find a user object from the user id in a python. Since I couldn't find anything in the Zope help I looked at the Membership tool from CMF. I looked as if I could try with this code: user_obj=self.acl_users.getUser(uname) But get an error saying that the user_obj is None (and therefor it can not find the object). Uname contains the correct Username ... Anyone who know how to get this object ??? Regards,