cv2.waitKey()

Python: cv2.waitKey([delay]) → retval

Parameters: 
delay – Delay in milliseconds. 0 is the special value that means “forever”.

The function waitKey waits for a key event infinitely (when delay <= 0 ) or for delay milliseconds, when it is positive. 
It returns the code of the pressed key or -1 if no key was pressed before the specified time had elapsed.

Note! The function only works if there is at least one HighGUI window created and the window is active. If there are several HighGUI windows, any of them can be active.

import cv2

def test():
    lena = cv2.imread('lena.jpg')
    cv2.imshow('image', lena)
    cv2.waitKey(0)
    print "I'm done"

if __name__ == '__main__':
    test()

import cv2

def test():
    lena = cv2.imread('lena.jpg')
    cv2.imshow('image', lena)
    keycode = cv2.waitKey(1000)
    print keycode
    print "I'm done"

if __name__ == '__main__':
    test()

97
I'm done

-1
I'm done

import cv2

def test():
    lena = cv2.imread('lena.jpg')
    while True:
        cv2.imshow('image', lena)
        if cv2.waitKey(1) & 0xFF == ord('q'):
            print "I'm done"
            break;

if __name__ == '__main__':
    test()

On some systems, waitKey() may return a value that encodes more than just the ASCII keycode. 
On all systems, we can ensure that we extract just the ASCII keycode by reading the last byte from the return value like this: 
keycode = cv2.waitKey(1)
if keycode != -1: 
  keycode &= 0xFF