title: Pandas GroupBy list values in a column of lists and find their mean
body: <p>I have a large pandas dataframe (1M rows) with the following format:</p>
  
  <pre><code>data = {
      'names': {0: ['Lily', 'Kerry', 'Mona'], 1: ['Kerry', 'Mona'], 2: ['Mona']},
      'sentiment': {0: 10, 1: 2, 2: 0}
  }
  df = pd.DataFrame(data)
  df
  
                   names  sentiment
  0  [Lily, Kerry, Mona]         10
  1        [Kerry, Mona]          2
  2               [Mona]          0
  </code></pre>
  
  <p>I would like to compute the average sentiment for each unique name in the names column, resulting in the following:</p>
  
  <pre><code>   names  sentiment
  0   Lily         10
  1  Kerry          6
  2   Mona          4
  </code></pre>
  
  <p>The number of unique names is extremely long so efficiency is important</p>