title: Why can't objgraph capture the growth of np.array()?
body: <p>See the code:</p>
  
  <pre><code>import objgraph
  import numpy as np
  objgraph.show_growth()
  j = 20
  y = []
  for i in range(5):
      for l in range(j):
          y.append(np.array([np.random.randint(500),np.random.randint(500)]))
      print 'i:',i
      objgraph.show_growth()
      print '___'
      #objgraph.show_most_common_types(limit=100)
      j += 1
  </code></pre>
  
  <p>the result is:</p>
  
  <pre><code>i: 1
  wrapper_descriptor 1596 +3
  weakref 625 +1
  dict 870 +1
  method_descriptor 824 +1
  i: 2
  i: 3
  i: 4
  </code></pre>
  
  <p>For the 2,3 and 4 epoch, it shows nothing growing. But it should show that the number of numpy.array grows</p>