title: Python ValueError: not enough values to unpack (expected 3, got 2)
body: <p>I am getting python for,</p>
  
  <h2>Code</h2>
  
  <pre><code>_, threshold = cv2.threshold(gray_roi, 3, 255, cv2.THRESH_BINARY_INV)
  _, contours, _ = cv2.findContours(threshold, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
  contours = sorted(contours, key=lambda x: cv2.contourArea(x), reverse=True)
  </code></pre>
  
  <h2>Error (2nd line)</h2>
  
  <pre><code>Traceback (most recent call last):
    File "/Users/hissain/PycharmProjects/hello/hello.py", line 17, in &lt;module&gt;
      _, contours, _ = cv2.findContours(threshold, cv2.RETR_TREE, cv2.CHAIN_APPROX_SIMPLE)
  ValueError: not enough values to unpack (expected 3, got 2)
  </code></pre>
  
  <p><strong>How to resolve it?</strong></p>