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?"

Start by reproducing the reveal_type results in airflow/www/views.py:850 and the TYPE_CHECKING attempt around line 115. Review how Flask's flask.globals types and stubgen handle third-party attributes. Done means the issue has a documented, maintainable approach or a clearly scoped type-checker feature for typing such attributes without a full replacement stub.