title: Python numpy array of numpy arrays
body: <p>I've got a problem on creating a numpy array of numpy arrays.
  I would create it in a loop:</p>
  
  <pre><code>a=np.array([])
  while(...):
     ...
     b= //a numpy array generated
     a=np.append(a,b)
     ...
  </code></pre>
  
  <p>Desired result:</p>
  
  <pre><code>[[1,5,3], [9,10,1], ..., [4,8,6]]
  </code></pre>
  
  <p>Real result:</p>
  
  <pre><code>[1,5,3,9,10,1,... 4,8,6]
  </code></pre>
  
  <p>Is it possible? I don't know the final dimension of the array, so I can't initialize it with a fixed dimension. </p>