title: Python opencv 3 SIFT feature extraction
body: <p>I want to change the following two commands which are written in opencv 2.3 .</p>
  
  <pre><code>fea_det=cv2.FeatureDetector_create("SIFT")
  des_ext=cv2.DescriptorExtractor_create("SIFT")
  </code></pre>
  
  <p>In opencv 3, I know that there is a command which create SIFT, so </p>
  
  <pre><code>fea_det=cv2.xfeatures2d.SIFT_create()
  </code></pre>
  
  <p>But what should I use for des_ext ? I am asking that what is the equivalent code of "<code>cv2.DescriptorExtractor_create("SIFT")</code>" in opencv 3?</p>