title: OpenCV cannot open *.mp4 file
body: <p>I am trying open <code>*.mp4</code> file with <code>opencv</code>. I am trying with following code but I am unable to do this. How can solve this issue? </p>
  
  <pre><code>import cv2
  cap = cv2.VideoCapture("test.mp4")
  # cap = cv2.VideoCapture("test.avi") # it's also not working
  cap.isOpened() # Output: False
  </code></pre>
  
  <ul>
  <li><p>N.B: </p>
  
  <ul>
  <li>OS : ubuntu 16.04</li>
  <li>OpenCv version: 3.3.0</li>
  </ul></li>
  </ul>