title: How to pick out all non-NULL value from multiple columns in Python Dataframe
body: <p>I had a DataFrame like below:</p>
  
  <pre><code>       column-a         column-b      column-c
  0          Nan             A              B
  1           A              Nan            C
  2           Nan            Nan            C
  3           A              B              C
  </code></pre>
  
  <p>I hope to create a new column-D to capture all non-NULL values from column A to C:</p>
  
  <pre><code>        column d
  0        A,B
  1        A,C
  2        C
  3        A,B,C
  </code></pre>
  
  <p>Thanks!</p>