title: Pandas replace column value under condition
body: <p>I have the following dataframe:</p>
  
  <pre><code>df = pd.DataFrame({'A' : [1., 2., 3., 4.], 'B' : [4., 3., 2., 1.], 'C' : [pd.NaT,8,pd.NaT,0]})
  df
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/mjRUC.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/mjRUC.png" alt="enter image description here"></a></p>
  
  <p>And I would like to change the column B to the value of C everytime that C is different from NaT. The final result would be the following:</p>
  
  <p><a href="https://i.stack.imgur.com/La2EV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/La2EV.png" alt="enter image description here"></a></p>