title: pandas fillna specific columns based on condition
body: <p>Below is an example of my DF</p>
  <pre><code>col1      col2     col3    col4 
  Comp1     False    01St    97
  Comp2     False    02St    97
  Comp3     True     03St    97
  Comp4     False    04St    97
  </code></pre>
  <p>The Aim would be to fillna col3 &amp; col4 when col2 == True</p>
  <p>Below is what I've tried so far</p>
  <pre><code>cols = df.columns.tolist()[-2]
  df[cols] = np.where(df.col2 == True, np.nan, df[cols])
  </code></pre>
  <p>The expected ouput</p>
  <pre><code>col1      col2     col3    col4 
  Comp1     False    01St    97
  Comp2     False    02St    97
  Comp3     True     NaN     NaN
  Comp4     False    04St    97
  </code></pre>