title: Pandas DataFrame select the specific columns with NaN values
body: <p>I have a two-column DataFrame, I want to select the rows with <code>NaN</code> in either column.     </p>
  
  <p>I used this method <code>df[ (df['a'] == np.NaN) | (df['b'] == np.NaN) ]</code><br>
  However it returns an empty answer. I don't know what's the problem</p>