title: pandas dataframe how to ffill nan columns when number of nan differ between rows
body: <p>I have a dataframe, when every row might have some nan in the last columns:</p>
  <pre><code>df = 
  A B C  D   E   F
  1 2 3 nan nan nan
  1 2 3  4  nan nan
  1 2 3  4   5   6 
  </code></pre>
  <p>I want to run ffill per row, to get:</p>
  <pre><code>df = 
  A B C  D  E F
  1 2 3  3  3 3
  1 2 3  4  4 4
  1 2 3  4  5 6 
  </code></pre>
  <p>How can I do it?</p>
  <p>Thanks</p>