title: Perform mathematical operation on a matching columns from 2 data frames: Python Pandas
body: <p>I have these two data frames with different row indices but some columns are same.
  What I want to do is to a get a data frame that sums the numbers of the two data frames with same column names</p>
  
  <pre><code>df1 = pd.DataFrame([(1,2,3),(3,4,5),(5,6,7)], columns=['a','b','d'], index = ['A', 'B','C','D'])
  df1
     a  b  d
  A  1  2  3
  B  3  4  5
  C  5  6  7
  
  df2 = pd.DataFrame([(10,20,30)], columns=['a','b','c'])
  
  df2
      a   b   c
  0  10  20  30
  </code></pre>
  
  <p>Output dataframe:</p>
  
  <pre><code>    a   b  d
  A  11  22  3
  B  13  24  5
  C  15  16  7
  </code></pre>
  
  <p>Whats the best way to do this?   .add() doesn't seem to work with data frames with different indices. </p>