code: def with_username(username, password):
      
      user = models.AuthUser.get_by_username(username)
      return _user(user) if user and user.password_matched(password) else None
docs: To authenticate a user with user and password
      *** authenticate doesn't create a session. To create a session, 
      use login_user
      :param username: 
      :param password: 
      :return: UserModel