title: Pandas/Numpy Get matrix from column of arrays
body: <p>I have a pandas dataframe with a column of lists.</p>
  
  <p>df:</p>
  
  <pre><code>    inputs
  0   [1, 2, 3]
  1   [4, 5, 6]
  2   [7, 8, 9]
  3   [10, 11, 12]
  </code></pre>
  
  <p>I need the matrix</p>
  
  <pre><code>array([[ 1,  2,  3],
        [ 4,  5,  6],
        [ 7,  8,  9],
        [10, 11, 12]])
  </code></pre>
  
  <p>An efficient way to do this?</p>
  
  <p>Note: When I try <code>df.inputs.as_matrix()</code> the output is</p>
  
  <pre><code>array([[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12]], dtype=object)
  </code></pre>
  
  <p>which has shape <code>(4,)</code>, not <code>(4,3)</code> as desired.</p>