title: numpy array: IndexError: too many indices for array
body: <p>This works:</p>
  
  <pre><code>&gt;&gt;&gt; a = np.array([[1,2,3,4], [5,6,7,8], [9,10,11,12]])
  &gt;&gt;&gt; a[: , 2]
  array([ 3,  7, 11])
  </code></pre>
  
  <p>This doesn't</p>
  
  <pre><code>&gt;&gt;&gt; a = np.array([[1,2,3,4], [5,6,7,8], [9,10,11]])
  &gt;&gt;&gt; a[:,2]
  Traceback (most recent call last):
    File "&lt;stdin&gt;", line 1, in &lt;module&gt;
  IndexError: too many indices for array
  </code></pre>
  
  <p>Why so ?</p>