title: Copy values from one column to another using Pandas
body: <p>I have a DataFrame with two columns X and Y:</p>
  
  <pre><code>   X      Y
  0  0  111.0
  1  3    NaN
  2  1    NaN
  3  1  112.0
  4  2  113.0
  5  2    NaN
  6  3  114.0
  7  3  115.0
  8  1    NaN
  9  2  116.0
  </code></pre>
  
  <p>I want to copy in Y only the values of X that correspond to the rows where a NaN is. The expected result should look like this</p>
  
  <pre><code>   X    Y
  0  0  111
  1  3    3
  2  1    1
  3  1  112
  4  2  113
  5  2    2
  6  3  114
  7  3  115
  8  1    1
  9  2  116
  </code></pre>
  
  <p>Is there any way to achieve this?</p>