title: NaN in mapper - name 'nan' is not defined
body: <p>I do as below</p>
  
  <pre><code>mapper = {'a': 'b', 'c': nan, 'd': 'e',  nan : nan}
  df['b'] = [ mapper[x] for x in df['a'] ]
  df['b'].value_counts()
  </code></pre>
  
  <p>and</p>
  
  <pre><code>NameError                                 Traceback (most recent call last)
  &lt;ipython-input-48-3862b2347ce7&gt; in &lt;module&gt;()
  NameError: name 'nan' is not defined
  </code></pre>
  
  <p>What's wrong? Is a mistake in coding or in file? </p>