title: Pandas DataFrame merge summing column
body: <p>I'm trying to merge two <code>DataFrames</code> summing columns value.</p>
  
  <pre><code>&gt;&gt;&gt; print(df1)
     id name  weight
  0   1    A       0
  1   2    B      10
  2   3    C      10
  
  &gt;&gt;&gt; print(df2)
     id name  weight
  0   2    B      15
  1   3    C      10
  </code></pre>
  
  <p>I need to sum <code>weight</code> values during merging for similar values in the common column.</p>
  
  <pre><code>merge = pd.merge(df1, df2, how='inner')
  </code></pre>
  
  <p>So the output will be something like following.</p>
  
  <pre><code>   id name  weight
  1   2    B      25
  2   3    C      20
  </code></pre>