https://www.pyimagesearch.com/2018/05/28/ubuntu-18-04-how-to-install-opencv/
use (the latest opencv) e.g. opencv 4.2.0 instead of 4.0.0 in the above doc
to run python , type version number well. this method install opencv in python3.6, since I have multiple python versions so I need to type (need to fix it)
>python3.6
in python3.6 >>>import cv2 as cv
in python3.6 >>>print(cv.__version__)
If it fails
https://stackoverflow.com/questions/54147922/no-module-named-cv2-but-it-is-installed
---- dlib problem ,?????