title: How to replace multiple column values on a condition when-then-otherwise in python polars?
body: <p>I have a dataframe as follows:</p>
  <pre><code>df_ = pl.DataFrame({'r_num':['Yes','','Yes'],'pin':['Yes','',''],'fin':['','','']})
  </code></pre>
  <p><a href="https://i.stack.imgur.com/ptyXJ.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ptyXJ.png" alt="enter image description here" /></a></p>
  <p>Here I would like to find an observation which has r_num is YES, pin is Yes and fin is EMPTY. on meeting this condition r_num and pin should be filled in as EMPTY.</p>
  <pre><code>df_.with_columns(pl.when((pl.col('r_num')=='Yes') &amp;  (pl.col('pin')=='Yes') &amp; (pl.col('fin') !='Yes')
                          ).then(pl.col('r_num')=='').otherwise(pl.col('r_num')))
  </code></pre>
  <p><a href="https://i.stack.imgur.com/5ixVu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5ixVu.png" alt="enter image description here" /></a></p>
  <p>Here why r_num is getting filled up with false?</p>
  <p>Here is a syantax that i'm following. how to specify  multiple columns in <strong>then</strong> clause.</p>
  <p>expected output will be:</p>
  <p><a href="https://i.stack.imgur.com/YLmHU.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/YLmHU.png" alt="enter image description here" /></a></p>
  <p>Here is a syntax to implement the same in pandas:</p>
  <pre><code>df_.loc[(df_['r_num']=='Yes') &amp; (df_['pin']=='Yes') &amp; (df_['fin']!='Yes'),['r_num','pin']]=''
  ``
  </code></pre>