Error when using python-opencv from apt on Ubuntu 16.04.4

[xiangyann]

This post is for informative purpose as I've encountered the problem earlier on(and solved it).
The cv2(opencv) module is available on Ubuntu 16.04.4 as 2 possible ways(pip3 not tested)
python-opencv from apt
opencv-python from pip

The pip version should be used instead of the apt version as the latter will just generate a ~5.6k avi file that includes nothing, probably related to Issue 5