title: Groupby multiindex pandas series using agg to sum AND apply list
body: <p>I have two multiindex series</p>
  
  <p>series 1</p>
  
  <pre><code>Company Name     Product     Price        TransactionID
  Company A        Apple       10           T0001
  Company B        Grapes      20           T0002
                   Orange      30           T0003
  </code></pre>
  
  <p>series 2</p>
  
  <pre><code>Company Name     Product     Price        TransactionID
  Company A        Orange      10           T0004
                   Apple       20           T0005
  Company B        Orange      20           T0006
  </code></pre>
  
  <p>I want to form the series like below</p>
  
  <pre><code>Company Name     Product     Sum_price    TransactionID
  Company A        Orange      10           [T0004]
                   Apple       30           [T0001,T0005]
  Company B        Orange      50           [T0003,T0006]
                   Grapes      20           [T0002]
  </code></pre>
  
  <p>From <a href="https://stackoverflow.com/questions/38174155/group-dataframe-and-get-sum-and-count">Group dataframe and get sum AND count?</a> shows that I can use .agg to do sum and count at the same time, but how to apply .apply(list)  and sum together? Right now, I have applied the code <a href="https://stackoverflow.com/questions/29015479/pandas-adding-multiindex-series-dataframes-containing-lists">Pandas: adding multiindex Series/Dataframes containing lists</a> to form two series one is sum and another one is the one with transactionID. I think I can concat two series together, but I just want to see whether there is a better way to do it.</p>