[Example pdraw.py] How to return value from yuv_frame_cb(yuv_frame)

Refer to example olympe/pdraw.py at master · Parrot-Developers/olympe · GitHub, I need returned value from yuv_frame_cb function to outer function like this, I can only see printed value by pdraw.set_callbacks(raw_cb=yuv_frame_cb), so what should I do to return the needed value. Any suggestions are welcome.