title: how to find the unique non nan values in a numpy array?
body: <p>I would like to know if there is a clean way to handle nan in numpy.</p>
  
  <pre><code>my_array1=np.array([5,4,2,2,4,np.nan,np.nan,6])
  print my_array1
  #[  5.   4.   2.   2.   4.  nan  nan   6.]
  print set(my_array1)
  #set([nan, nan, 2.0, 4.0, 5.0, 6.0])
  </code></pre>
  
  <p>I would have thought it should return at most 1 nan value. Why does it return multiple nan values?
  I would like to know how many unique non nan values I have in a numpy array.</p>
  
  <p>Thanks</p>