      "\n",
      "class TestMergeSort(object):\n",
      "    def test_merge_sort(self):\n",
      "        print('Empty input')\n",
      "        data = []\n",
      "        sorted_data = merge_sort(data)\n",
      "        assert_equal(sorted_data, [])\n",
 
</s>
    "        print('None input')\n",
    "        data = None\n",
    "        sorted_data = merge_sort(data)\n",
    "        assert_equal(sorted_data, None)\n",
    "\n",
      "\n",
      "class TestMergeSort(object):\n",
      "    def test_merge_sort(self):\n",
    "        print('None input')\n",
    "        data = None\n",
    "        sorted_data = merge_sort(data)\n",
    "        assert_equal(sorted_data, None)\n",
    "\n",
      "        print('Empty input')\n",
      "        data = []\n",
      "        sorted_data = merge_sort(data)\n",
      "        assert_equal(sorted_data, [])\n",
keep keep add keep keep keep keep
 Polish merge sort challenge and solution

Add None input test case.  Update space complexity discussion.
sorting_searching/merge_sort/merge_sort_solution.ipynb