          src = Stack()
          buff = Stack()
          dest = Stack()
  
          print('Test: NULL towers')
          hanoi(num_disks, None, None, None)
  
          print('Test: 0 disks')
          hanoi(num_disks, src, dest, buff)
          assert_equal(dest.pop(), None)
 
</s> remove     "        print('Test: NULL towers')\n",
 </s> add     "        print('Test: None towers')\n", </s> remove     "        print('Test: NULL towers')\n",
 </s> add     "        print('Test: None towers')\n", </s> remove         print('Test: find a NULL')
 </s> add         print('Test: find a None') </s> remove         print('Test: delete a NULL')
 </s> add         print('Test: delete a None') </s> remove     "        print('Test: find a NULL')\n",
 </s> add     "        print('Test: find a None')\n",
        print('Test: None towers')
          src = Stack()
          buff = Stack()
          dest = Stack()
  
        print('Test: None towers')
          hanoi(num_disks, None, None, None)
  
          print('Test: 0 disks')
          hanoi(num_disks, src, dest, buff)
          assert_equal(dest.pop(), None)
keep keep keep keep replace keep keep keep keep keep
 Changed discussions of NULL to None to be more Pythonic. 
stacks_queues/hanoi/test_hanoi.py