title: Replace int64 value with "None" based on condition of another column in pandas dataframe
body: <p>I have pandas dataframe</p>
  
  <p><a href="https://i.stack.imgur.com/CbJgM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/CbJgM.png" alt="[1]: https://i.stack.imgur.com/jWe9U.png"></a></p>
  
  <p>Conditions are:</p>
  
  <p>'If there is a value other than -1 in RPoints, then any 0 in kPoints should be treated as "None"</p>
  
  <p>'If there is a value other than -1 in RPoints, then any 0 in WPoints should be treated as "None"</p>
  
  <p>Datatypes are as:</p>
  
  <pre><code>kPoints         int64
  
  RPoints         int64
  
  WPoints         int64
  </code></pre>
  
  <p>How to replace -1 with 'None'?
  And I guess "None" is not going to be helpful for analysis , so how should I remove those rows containing 'None'?</p>