title: from a dataframe column check if a string is nan
body: <p>From a dataframe when I print <code>data['words'].values</code> I get,</p>
  
  <pre><code>['from' 'fairest' 'creatures' 'we' 'desire' 'increase' nan 'that' 'thereby']
  </code></pre>
  
  <p>When I loop through like this how do I identify if the value is nan?</p>
  
  <pre><code>for w in data['words'].values:
      check if w is nan ????
  </code></pre>