show image opencv python

import cv2
img = cv2.imread('/path_to_image/opencv-logo.png')
cv2.imshow('image',img)
cv2.waitKey(0)
cv2.destroyAllWindows()

# to use it in a loop
k = cv2.waitKey(0)
if k == 27:         # wait for ESC key to exit
    cv2.destroyAllWindows()
elif k == ord('s'): # wait for 's' key to save and exit
    cv2.imwrite('messigray.png',img)
    cv2.destroyAllWindows()

    import numpy as np
    import cv2
    import matplotlib.pyplot as plt
    %matplotlib inline # if you are running this code in jupyter notebook

    img = cv2.imread('/path_to_image/opencv-logo.png',0) # reads image 'opencv-logo.png' as grayscale
    plt.imshow(img, cmap='gray')

img = cv2.imread('img.jpg')