title: Python Numpy mask NaN not working
body: <p>I'm simply trying to use a masked array to filter out some <code>nan</code>entries.</p>
  
  <pre><code>import numpy as np
  # x = [nan, -0.35, nan]
  x = np.ma.masked_equal(x, np.nan)
  print x
  </code></pre>
  
  <p>This outputs the following:</p>
  
  <pre><code>masked_array(data = [        nan -0.33557216         nan],
           mask = False,
     fill_value = nan)
  </code></pre>
  
  <p>Calling <code>np.isnan()</code> on <code>x</code> returns the correct boolean array, but the mask just doesn't seem to work. Why would my mask not be working as I expect? </p>