title: Manipulating value in a column based on a rule
body: <p>I have 3 columns -<code>A, B and C</code> in a  pandas dataframe. What i want to do is, where ever <code>A is not null AND B|C are not null</code>, that row in <code>A</code> should be set to <code>null</code>.</p>
  
  <pre><code>if(dffinal['A'].loc[dffinal['A'].notnull()] &amp; 
      (dffinal['B'].loc[dffinal['B'].notnull()] | 
      dffinal['C'].loc[dffinal['C'].notnull()])):
      dffinal['A'] = np.nan
  </code></pre>
  
  <p>this is the error I'm getting: cannot do a non-empty take from an empty axes.</p>