title: List in column Python + Pandas
body: <p>I'm new to pandas and would like to analyse some data arranged like this:</p>
  
  <pre><code>label                     aa                        bb
  index
      0           [2, 5, 1, 4]          [x1, x2, y1, z1]
      1             [3, 3, 19]              [x3, x4, y2]
      2    [6, 4, 2, 8, 9, 10]  [y1, y2, z3, z4, x1,  w]
  </code></pre>
  
  <p>in which x1,x2,x3,x4 are of type M; y1,y2 are of type N; and z1,z2,z3,z4 are of type O. Note that data[2,'bb'] is w, which does not belong to any type. This relationship is defined in mongodb as follows</p>
  
  <pre><code>{'_id' : ObjectId(x1), type : 'M'}
  {'_id' : ObjectId(y1), type : 'N'}
  {'_id' : ObjectId(z1), type : 'O'}...
  db.data.find({'_id' : ObjectId(w)}) is null
  </code></pre>
  
  <p>The desired output would be like this:</p>
  
  <pre><code>label     sum_M   sum_N   sum_O
  index
      0         7       1       4
      1         6      19       0
      2         9      10      10
  </code></pre>
  
  <p>Does anyone know how to do this with pandas?</p>