      "        print('Test: None towers')\n",
      "        hanoi(num_disks, None, None, None)\n",
      "\n",
      "        print('Test: 0 disks')\n",
      "        hanoi(num_disks, src, dest, buff)\n",
      "        assert_equal(dest.pop(), None)\n",
      "\n",
      "        print('Test: 1 disk')\n",
      "        src.push(5)\n",
      "        hanoi(num_disks, src, dest, buff)\n",
 
</s> remove     "        hanoi(num_disks, src, dest, buff)\n",
 </s> add     "        hanoi.move_disks(num_disks, src, dest, buff)\n", </s> remove     "        hanoi(num_disks, None, None, None)\n",
 </s> add     "        assert_raises(TypeError, hanoi.move_disks, num_disks, None, None, None)\n", </s> remove     "        hanoi(num_disks, None, None, None)\n",
 </s> add     "        assert_raises(TypeError, hanoi.move_disks, num_disks, None, None, None)\n", </s> remove     "        hanoi(num_disks, src, dest, buff)\n",
 </s> add     "        hanoi.move_disks(num_disks, src, dest, buff)\n", </s> remove     "        hanoi(num_disks, src, dest, buff)\n",
 </s> add     "        hanoi.move_disks(num_disks, src, dest, buff)\n",
    "        hanoi.move_disks(num_disks, src, dest, buff)\n",
      "        print('Test: None towers')\n",
      "        hanoi(num_disks, None, None, None)\n",
      "\n",
      "        print('Test: 0 disks')\n",
    "        hanoi.move_disks(num_disks, src, dest, buff)\n",
      "        assert_equal(dest.pop(), None)\n",
      "\n",
      "        print('Test: 1 disk')\n",
      "        src.push(5)\n",
      "        hanoi(num_disks, src, dest, buff)\n",
keep keep keep keep replace keep keep keep keep keep
 Update hanoi challenge (#121)

Update constraints and test cases.  Move implementation to a class.  Update tests. 
recursion_dynamic/hanoi/hanoi_challenge.ipynb