title: Summing Two DataFrames by Index
body: <p>I have the following  </p>
  
  <pre><code>df1 = pd.DataFrame([1, 1, 1, 1, 1], index=[ 1, 2, 3, 4 ,5 ], columns=['A'])  
  df2 = pd.DataFrame([ 1, 1, 1, 1, 1], index=[ 2, 3, 4, 5, 6], columns=['A']) 
  </code></pre>
  
  <p>I want to return the DataFrame which will be the sum of the two for each row:  </p>
  
  <pre><code>df = pd.DataFrame([ 1, 2, 2, 2, 2, 1], index=[1, 2, 3, 4, 5, 6], columns=['A'])  
  </code></pre>
  
  <p>of course, the idea is that I don't know what the actual indices are, so the intersection could be empty and I'd get a concatenation of both DataFrames.</p>