donnemartin/interactive-coding-challenges · error · TypeError

data cannot be None

Error message

data cannot be None

What it means

Raised by InsertionSort.sort when data is None. The very next line calls len(data), so None is rejected explicitly with a descriptive TypeError rather than the implicit 'object of type NoneType has no len()'.

Source

Thrown at sorting_searching/insertion_sort/insertion_sort_solution.ipynb:103

  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class InsertionSort(object):\n",
    "\n",
    "    def sort(self, data):\n",
    "        if data is None:\n",
    "            raise TypeError('data cannot be None')\n",
    "        if len(data) < 2:\n",
    "            return data\n",
    "        for r in range(1, len(data)):\n",
    "            for l in range(r):\n",
    "                if data[r] < data[l]:\n",
    "                    temp = data[r]\n",
    "                    data[l+1:r+1] = data[l:r]\n",
    "                    data[l] = temp\n",
    "        return data"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test\n",
    "\n"
   ]

View on GitHub (pinned to 358f2cc604)

Solutions

1. Ensure the list is initialized to [] before passing
2. Coalesce: data = data or []
3. Have data-producing functions return empty collections on failure

Example fix

// before
sorted_data = InsertionSort().sort(records)
// after
records = records or []
sorted_data = InsertionSort().sort(records)

data = data if data is not None else []
InsertionSort().sort(data)

def is_list(x):
    return isinstance(x, list)

try:
    InsertionSort().sort(data)
except TypeError:
    data = []
    InsertionSort().sort(data)

When it happens

Trigger: Calling sort(None). Lists of length 0 or 1 are returned unchanged; only None raises.

Common situations: Sorting a list produced by a filter that returned None; data fetched from a source with a missing array; a variable declared but never assigned before the sort call.

Related errors

• data cannot be None
• num cannot be None
• max_num cannot be None
• val cannot be None
• n cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/301ba363ccadad21. Report an issue: GitHub.