This problem also occurred with me this morning, while i started leaning OpenCV, below solution worked for me, probably this will work for you people also:

--------------------------------------------------

import numpy as np

import cv2

img = np.zeros((512, 512, 3), np.uint8)

img= cv2.line(img, (0, 0), (511, 511), (255, 0, 0), 5)

cv2.imshow("Image", img)

cv2.waitKey(0)

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Basically, window with line disappears in mili-seconds, so we just need to **add a infinite wait key so that window does not get disappear automatically.

This problem also occurred with me this morning, while i started leaning OpenCV, below solution worked for me, probably this will work for you people also:

import numpy as np
np
import cv2
  
import cv2
 
img = np.zeros((512, 512, 3), np.uint8)
 
np.uint8)
img= cv2.line(img, (0, 0), (511, 511), (255, 0, 0), 5)5)
  
cv2.imshow("Image", img)
 
cv2.waitKey(0)
 
---------------------------------------------------img)
cv2.waitKey(0)