title: How to use pandas.DataFrame.mask for NaN
body: <p>I want to ignore NaN values in my selected dataframe columns when I want to normalize with <a href="http://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.normalize.html" rel="nofollow noreferrer"><code>sklearn.preprocessing.normalize</code></a>. Column example: </p>
  
  <pre><code>0     12.0
  1     12.0
  2      3.0
  3      NaN
  4      3.0
  5      3.0
  6      NaN
  7      NaN
  8      3.0
  9      3.0
  10     3.0
  11     4.0
  12    10.0
  </code></pre>