I wrote a freehand drawing tool using Qt, PySide, Python. See https://github.com/bootchk/freehandTool

Input is pointer (mouse) events, output is a QGraphicsPathItem (i.e. vector graphics.)

A component that could be adapted for a full app.

Doesn't teach much about Qt, but it is a common concern for drawing apps.

Uses fairly sophisticated algorithms from potrace. Shows use of Python generators in a filter pipe.

The moderators will no doubt bust this to a different forum.