title: Filling nan of one column with the values of another Python
body: <p>I have a dataset that has been merged together to fill <strong>missing values</strong> from one another.</p>
  
  <p>The problem is that I have some columns with missing data that I want to now fill with the values that aren't missing.</p>
  
  <p>The merged data set looks like this for an <strong>input</strong>:</p>
  
  <pre><code>Name         State       ID       Number_x      Number_y       Op_x       Op_y
  Johnson      AL          1        1             nan            1956       nan
  Johnson      AL          1        nan           nan            1956       nan
  Johnson      AL          2        1             nan            1999       nan
  Johnson      AL          2        0             nan            1999       nan
  Debra        AK          1A       0             nan            2000       nan
  Debra        AK          1B       nan           20             nan        1997
  Debra        AK          2        nan           10             nan        2009
  Debra        AK          3        nan           1              nan        2008
  .
  .
  </code></pre>
  
  <p>What I'd want for an <strong>output</strong> is this:</p>
  
  <pre><code>Name         State       ID       Number_x      Number_y     Op_x       Op_y
  Johnson      AL          1        1             1            1956       1956
  Johnson      AL          2        1             1            1999       1999
  Johnson      AL          2        0             0            1999       1999
  Debra        AK          1A       0             0            2000       2000
  Debra        AK          1B       20            20           1997       1997
  Debra        AK          2        10            10           2009       2009
  Debra        AK          3        1             1            2008       2008
  .
  .
  </code></pre>
  
  <p>So I want it so that all <code>nan</code> values are replaced by the associated  values in their columns - match <code>Number_x</code> to <code>Number_y</code> and <code>Op_x</code> to <code>Op_y</code>.</p>
  
  <p>One thing to note is that when there are <strong>two IDs that are the same</strong> sometimes their values will be different; like <code>Johnson</code> with <code>ID = 2</code> which has different numbers but the same op values. I want to keep these because I need to investigate them more.</p>
  
  <p>Also, if the row has two missing values for <code>Number_x</code> and <code>Number_y</code> I want to take that row out - like <code>Johnson</code> with <code>Number_x</code> and <code>Number_y</code> missing as a <code>nan</code> value.</p>