How can flask find the controller?

[Larz60+]

if you want to use the abbreviation, it must be defined.
I think that if you change line 5 in 1st listing to:

from application.Controller.app_ctrl import AppController as app_ctrl

you should be OK.

Or change line 26, 40, 47 and 68 to AppController...
example line 26:

verify_user_id = AppController.get_user(user_id)

but not both.