Anyone figured out how to create custom notifications? (script modding)

The text itself would presumably be translated into multiple languages and located with the other game text.

A 'Factory' in programming generally means a class that makes objects. So I would guess that BILL_ARRIVAL_NOTIFICATION in this code is a class that is called inside the _send_notification() function to create the actual 'Tunable' notification objects. I suspect the factory class name is passed into the factory's create method to make the objects distinct.

Probably there is a tunable datafile somewhere that has a reference to 'BILL_ARRIVAL_NOTIFICATION' which in turn points to the language data, either that or 'BILL_ARRIVAL_NOTIFICATION' goes directly to the language files - I'm not sure exactly how the localisation fits in.

So. To make a new notification I would write:

MY_NEW_NOTIFICATION = UiDialogNotification.TunableFactory(description='\n  A test notification.\n

self._send_notification(self.MY_NEW_NOTIFICATION)

Then find whatever datafiles contain the localisation and see if there is a direct reference to BILL_ARRIVAL_NOTIFICATION in there, if not I would explore the gamedata package with the Datafile tool: (http://www.modthesims.info/showthread.php?t=531876) and see if you can find a reference to the notification somewhere in there that then refers to the localisation data. Wherever it is, eventually you'll need to add the text into the localisation files.

Notifications is indeed a tunable instance and the description can be found under Snippets in the Modding Documentation.