title: Pandas fillna() not working on DataFrame slices
body: <p>Pandas <code>fillna</code> is not working on DataFrame slices, here is an example</p>
  
  <pre><code>df = pd.DataFrame([[np.nan, 2, np.nan, 0],
                  [3, 4, np.nan, 1],
                  [np.nan, np.nan, np.nan, 5],
                  [np.nan, 3, np.nan, 4]],
                  columns=list('ABCD'))
  df[["A", 'B']].fillna(0, inplace=True)
  </code></pre>
  
  <p>the <code>DataFrame</code> doesn't change</p>
  
  <pre><code>    A   B   C   D
  0   NaN 2.0 NaN 0
  1   3.0 4.0 NaN 1
  2   NaN NaN NaN 5
  3   NaN 3.0 NaN 4
  </code></pre>
  
  <p>in contrast</p>
  
  <pre><code>df["A"].fillna(0, inplace=True)
  </code></pre>
  
  <p>and</p>
  
  <pre><code>df.fillna(0, inplace=True)
  </code></pre>
  
  <p>work fine. </p>
  
  <p>Is this a bug or does it work as intended? 
  Thx in advance.</p>
  
  <p>P.S. <a href="https://stackoverflow.com/questions/38134012/pandas-dataframe-fillna-only-some-columns-in-place">this</a> question asks <strong>how</strong> to use <code>fillna</code> on a slice, as for my question, it concerns <strong>why</strong> the above does'n work.
  The answer is in @heena-bawa answers comment section.</p>