title: Pandas DFs and find missing columns filled with NAN in each
body: <p>df, df2 and df3 as an example with some overlapping columns. </p>
  
  <pre><code>    &gt;&gt;&gt; df
     A  B  C
  0  3  4  3
  1  1  1  4
  2  4  4  4
  3  2  3  2
  4  1  0  3
  
  &gt;&gt;&gt; df2
     A  B  C  D
  0  1  0  2  2
  1  3  3  2  3
  2  0  0  1  1
  3  0  0  1  1
  4  1  0  2  2
  
  &gt;&gt;&gt; df3
  
     A  B  C  D  F
  0  2  2  0  1  1
  1  4  3  3  0  3
  2  0  4  0  0  0
  3  0  2  2  1  0
  4  4  2  4  1  1
  </code></pre>
  
  <p>find those missing columns and added them to each df with NAN values</p>
  
  <p>expected results:</p>
  
  <pre><code>    &gt;&gt;&gt; df
     A  B  C D  F
  0  3  4  3 nan nan
  1  1  1  4 nan nan
  2  4  4  4 nan nan
  3  2  3  2 nan nan
  4  1  0  3 nan nan
  
  &gt;&gt;&gt; df2
     A  B  C  D F
  0  1  0  2  2 nan
  1  3  3  2  3 nan
  2  0  0  1  1 nan
  3  0  0  1  1 nan
  4  1  0  2  2 naa
  </code></pre>
  
  <p>df3 as it is</p>