Install OpenCV 4 for Python on Ubuntu Linux

How to install OpenCV 4 for Python on Ubuntu Linux 20.04.

Commands used in the video:
sudo apt update
sudo apt upgrade
sudo apt install python3-venv python3-pip python3-tk
python3 -m venv work
source work/bin/activate
pip install opencv-python

[irengerey]

Traceback (most recent call last):
File "ocv_demo.py", line 8, in <module>
gray=cv2.cvtColor(img, cv2.COLOR_BGR2GRAVY)
AttributeError: module 'cv2.cv2' has no attribute 'COLOR_BGR2GRAVY