title: How to group dataframe rows into list in pandas groupby
body: <p>I have a pandas data frame <code>df</code> like:</p>
  
  <pre><code>a b
  A 1
  A 2
  B 5
  B 5
  B 4
  C 6
  </code></pre>
  
  <p>I want to <strong>group by the first column and get second column as lists in rows</strong>:</p>
  
  <pre><code>A [1,2]
  B [5,5,4]
  C [6]
  </code></pre>
  
  <p>Is it possible to do something like this using pandas groupby?</p>