title: Calculate the sum of the first n rows for each group
body: <p>What I want to do is group by column A and then take the sum of first two rows, then assign that value as a new column. Example  below:</p>
  <p>DF:</p>
  <pre><code>ColA   ColB
   AA      2
   AA      1
   AA      5
   AA      3
   BB      9
   BB      3
   BB      2
   BB      12
   CC      0
   CC      10
   CC      5
   CC      3
  </code></pre>
  <p>Desired DF:</p>
  <pre><code>ColA   ColB    NewCol
   AA      2       3
   AA      1       3
   AA      5       3 
   AA      3       3
   BB      9       12
   BB      3       12
   BB      2       12
   BB      12      12
   CC      0       10
   CC      10      10 
   CC      5       10
   CC      3       10
  </code></pre>
  <p>For AA, it looks at ColB and take the sum of the first two rows and assigns that summed value to newCol. I've tried this by creating a dictionary by looping through the unique ColA values, creating a  subset dataframe of the first two rows, summing, then populating the dictionary with values. Then mapping the dictionary back - but my dataframe is VERY big and it takes forever. Any ideas?</p>
  <p>Thank you!</p>