title: Pandas DataFrame column values in to list
body: <p>I have a pandas <code>DataFrame</code> like following</p>
  
  <pre><code>                          clusters
  0                              [4]
  1                  [9, 14, 16, 19]
  2           [6, 7, 10, 17, 18, 20]
  3  [1, 2, 3, 5, 8, 11, 12, 13, 15]
  </code></pre>
  
  <p>I need to get only the integer values in the cluster column separately. Like following(This can be four lists no need of having another <code>DataFrame</code>)</p>
  
  <pre><code>0                              4
  1                  9, 14, 16, 19
  2           6, 7, 10, 17, 18, 20
  3  1, 2, 3, 5, 8, 11, 12, 13, 15
  </code></pre>
  
  <p>I tried different things. Could not achieve the expected output.</p>
  
  <pre><code>In [36]: clustlist = list(firstclusters.clusters.values)
  Out[36]:   
      [array([4]), array([ 9, 14, 16, 19]), array([ 6,  7, 10, 17, 18, 20]), array([ 1,  2,  3,  5,  8, 11, 12, 13, 15])]
  
  In [37]: np.ravel(clustlist)
  Out[37]:
      [array([4]) array([ 9, 14, 16, 19]) array([ 6,  7, 10, 17, 18, 20])
       array([ 1,  2,  3,  5,  8, 11, 12, 13, 15])]
  
  In [38]: np.hstack(clustlist)
  Out[38]:
      [ 4  9 14 16 19  6  7 10 17 18 20  1  2  3  5  8 11 12 13 15]
  </code></pre>