title: Pandas overlay one dataframe on another
body: <p>I have two data frames which have identical columns and the exact same number of rows  </p>
  
  <p>Data frame 1 has the original data and data frame 2 contains the fields which have changed (changed fields contain the new value, unchanged fields are nan).</p>
  
  <p>I want to essentially overlay the "new data" in df 2 over the data in df 1 and get the result below.</p>
  
  <pre><code>df1:
  
  Key   a    b
  
  123   6    1
  124   7    6
  125   3    5
  
  df2:
  
  Key   a    b
  
  123   nan  nan
  124   8    nan
  125   nan  4
  
  Result df:
  
  Key   a    b
  
  123   6    1
  124   8    6
  125   3    4
  </code></pre>