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.