title: what does it mean [ WARN:0] terminating async callback?
body: <p>i use videocapture in python by cmd but i got this message when it return</p>
  
  <p>c</p>
  
  <p>[ WARN:0] terminating async callback</p>
  
  <p>d</p>
  
  <p>this is my code</p>
  
  <pre><code>import numpy as np
  import cv2
  import sys
  
  cap = cv2.VideoCapture(0)
  
  while(cap.isOpened()):
      ret, frame = cap.read()
      gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
      cv2.imshow('frame',gray)
      if cv2.waitKey(1) &amp; 0xFF == ord('q'):
          print('a')
          break
  print('c')
  cap.release()
  print('d')
  cv2.destroyAllWindows()
  </code></pre>
  
  <p>my python version 3.7.0</p>