title: After Grouping ..Sum of a column values based on the groups in pandas dataframe
body: <p>After grouping the column of the pandas dataframe how to do sum of another column of a dataframe based on the groups created.</p>
  
  <p><code>df = df.groupby(by=['First'])</code></p>
  
  <p>My dataframe:</p>
  
  <pre><code>First  Second
  
  A      5
         6
         10
  B      4
         5
  </code></pre>
  
  <p>expected dataframe:</p>
  
  <pre><code>First  Second
  
  A      21
  B      9
  </code></pre>