title: Converting NumPy array to a set takes too long
body: <p>I'm trying to execute the following</p>
  
  <pre><code>from numpy import *
  x = array([[3,2,3],[711,4,104],.........,[4,4,782,7845]])  # large nparray
  for item in x:
      set(item)
  </code></pre>
  
  <p>and it takes very long compared to:</p>
  
  <pre><code>x = array([[3,2,3],[711,4,104],.........,[4,4,782,7845]])  # large nparray
  for item in x:
      item.tolist()
  </code></pre>
  
  <p>Why does it take much longer to convert a NumPy array to a <code>set</code> than to a <code>list</code>?
  I mean basically both have complexity <code>O(n)</code>? </p>