title: fill up empty values with same value of another column in pandas dataframe
body: <p>i have a pandas dataframe like the following:</p>
  
  <p><a href="https://i.stack.imgur.com/skdgd.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/skdgd.png" alt="enter image description here"></a></p>
  
  <p>How do I fill up the empty cells with same policy numbers for same product type as they exist?</p>
  
  <p>Any suggestion would be very much appreciated. Thank you</p>
  
  <p>Sorry for the confusion, I am adding my sample dataframe now:</p>
  
  <blockquote>
    <blockquote>
      <p>sample=[{'POLICY NUMBER':' ','PRODUCT TYPE':'MED'},{'POLICY NUMBER':' ','PRODUCT TYPE':'MED'},{'POLICY NUMBER':'433M49763','PRODUCT TYPE':'MED'},{'POLICY NUMBER':'433M86968','PRODUCT TYPE':'MED'},{'POLICY NUMBER':' ','PRODUCT TYPE':'TED'},{'POLICY NUMBER':'566D158635 ','PRODUCT TYPE':'TED'},{'POLICY NUMBER':'655D158635','PRODUCT TYPE':'TED'},{'POLICY NUMBER':'789D158635','PRODUCT TYPE':'TED'}]</p>
      
      <p>pd.DataFrame(sample)</p>
    </blockquote>
  </blockquote>
  
  <p>please note that the empty cells have " " in them too, they are not NaN across the whole dataframe</p>
  
  <p>Adding to the question above. If I have the altered dataframe as above. How do i get to the following dataframe:</p>
  
  <p><a href="https://i.stack.imgur.com/r8ogs.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/r8ogs.png" alt="enter image description here"></a></p>