How to draw a rectangle around a region of interest in python

As the other answers said, the function you need is cv2.rectangle(), but keep in mind that the coordinates for the bounding box vertices need to be integers if they are in a tuple, and they need to be in the order of (left, top) and (right, bottom). Or, equivalently, (xmin, ymin) and (xmax, ymax).

please don't try with the old cv module, use cv2:

import cv2

cv2.rectangle(img, (x1, y1), (x2, y2), (255,0,0), 2)


x1,y1 ------
|          |
|          |
|          |
--------x2,y2

[edit] to append the follow-up questions below:

cv2.imwrite("my.png",img)

cv2.imshow("lalala", img)
k = cv2.waitKey(0) # 0==wait forever