title: How to pass another entire column as argument to pandas fillna()
body: <p>I would like to fill missing values in one column with values from another column, using <code>fillna</code> method. </p>
  
  <p>(I read that looping through each row would be very bad practice and that it would be better to do everything in one go but I could not find out how to do it with <code>fillna</code>.)</p>
  
  <p>Data before:</p>
  
  <pre><code>Day  Cat1  Cat2
  1    cat   mouse
  2    dog   elephant
  3    cat   giraf
  4    NaN   ant
  </code></pre>
  
  <p>Data after:</p>
  
  <pre><code>Day  Cat1  Cat2
  1    cat   mouse
  2    dog   elephant
  3    cat   giraf
  4    ant   ant
  </code></pre>