Add typed attribute to third party class.

Feature

I would like to be able to add a (typed) attribute to a third-party class instance.

To give a specific example, Flask's current_app, and Flask-AppBuilder. Some example code:

    def my_view(self):
        reveal_type(current_app.appbuilder)
        if current_app.appbuilder.sm.has_access(...):
            ...

The reveal_type here shows Any: airflow/www/views.py:850: note: Revealed type is 'Any'

Things I tried:

1. To add a typed attribute myself:

   if TYPE_CHECKING:
       current_app.appbuilder = AppBuilder(None)
       reveal_type(current_app.appbuilder)
   

   Didn't work: airflow/www/views.py:115: note: Revealed type is 'Any'

2. A partial type stub file

   I could use stubgen etc to create a stub file for all of flask and then edit the type in flask.globals, but this presents a maintenance burden that I don't want (keeping it in sync).

   I searched for any way I could to have a "partial" stub file -- if I could have a stubs/flask/globals.pyi with out neededing all the rest of the type stubs for this module, but couldn't get it working - likely not supported.

Pitch

I don't know what the feature would look like, but for the flask ecosystem it is fairly common to add properties on to the global flask object, or the g global variable, and I would love to be able to type-check my use of them, without needing to cast it at each use.

This issue is mostly a request for help/ "is there any sensible way of doing this?"

Inizia riproducendo i risultati di reveal_type in airflow/www/views.py:850 e il tentativo con TYPE_CHECKING intorno alla riga 115. Esamina come i tipi di Flask's flask.globals e stubgen gestiscono gli attributi di terze parti. Il lavoro sarà considerato completato quando l'issue avrà un approccio documentato e manutenibile oppure una funzionalità del type-checker chiaramente circoscritta per tipizzare tali attributi senza uno stub sostitutivo completo.