title: Apply custom functions to groupby pandas
body: <p>I know there are questions/answers about how to use custom function for groupby in pandas but my case is slightly different.</p>
  <p>My data is</p>
  <pre><code>  group_col       val_col
  0         a    [1, 2, 34]
  1         a        [2, 4]
  2         b  [2, 3, 4, 5]
  
  data = {'group_col': {0: 'a', 1: 'a', 2: 'b'}, 'val_col': {0: [1, 2, 34], 1: [2, 4], 2: [2, 3, 4, 5]}}
  df = pd.DataFrame(data)
  </code></pre>
  <p>What I am trying to do is to group by <code>group_col</code>, then sum up the length of lists in <code>val_col</code> for each group. My desire output is</p>
  <pre><code>a     5
  b     4
  </code></pre>
  <p>I wonder I can do this in pandas?</p>