title: Setting elements to None in pandas dataframe
body: <p>I'm not sure why this happens</p>
  
  <pre><code>&gt;&gt;&gt; df = pd.DataFrame(np.arange(15).reshape(5,3),columns=list('ABC'))
  &gt;&gt;&gt; df
      A   B   C
  0   0   1   2
  1   3   4   5
  2   6   7   8
  3   9  10  11
  4  12  13  14
  </code></pre>
  
  <p>Assign <code>None</code> to elements in last row turns it into <code>NaN NaN NaN</code>:</p>
  
  <pre><code>&gt;&gt;&gt; df.ix[5,:] = None
  &gt;&gt;&gt; df
      A   B   C
  0   0   1   2
  1   3   4   5
  2   6   7   8
  3   9  10  11
  4  12  13  14
  5 NaN NaN NaN
  </code></pre>
  
  <p>Change two element in last column to 'nan'</p>
  
  <pre><code>&gt;&gt;&gt; df.ix[:1,2] = 'nan'
  &gt;&gt;&gt; df
      A   B    C
  0   0   1  nan
  1   3   4  nan
  2   6   7    8
  3   9  10   11
  4  12  13   14
  5 NaN NaN  NaN
  </code></pre>
  
  <p>Now last row becomes <code>NaN NaN  None</code></p>
  
  <pre><code>&gt;&gt;&gt; df.ix[5,:] = None
  &gt;&gt;&gt; df
      A   B     C
  0   0   1   nan
  1   3   4   nan
  2   6   7     8
  3   9  10    11
  4  12  13    14
  5 NaN NaN  None
  </code></pre>