donnemartin/interactive-coding-challenges · error · TypeError

data cannot be None

Error message

data cannot be None

What it means

SelectionSort.sort raises TypeError('data cannot be None') as the first statement; the algorithm then indexes data[i] and compares elements, so None input is rejected up front with a clear message. Empty and single-element lists are valid and returned unchanged.

Source

Thrown at sorting_searching/selection_sort/selection_sort_solution.ipynb:105

  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class SelectionSort(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 i in range(len(data) - 1):\n",
    "            min_index = i\n",
    "            for j in range(i + 1, len(data)):\n",
    "                if data[j] < data[min_index]:\n",
    "                    min_index = j\n",
    "            if data[min_index] < data[i]:\n",
    "                data[i], data[min_index] = data[min_index], data[i]\n",
    "        return data\n",
    "\n",
    "    def sort_iterative_alt(self, data):\n",
    "        if data is None:\n",
    "            raise TypeError('data cannot be None')\n",
    "        if len(data) < 2:\n",
    "            return data\n",
    "        for i in range(len(data) - 1):\n",
    "            self._swap(data, i, self._find_min_index(data, i))\n",

View on GitHub (pinned to 358f2cc604)

Solutions

1. Initialize the variable to [] or guard with 'if data is not None' before sorting
2. Fix the upstream producer so it returns a list (or raises its own clear error) instead of None
3. Substitute an empty list when None means no data: sort(data or [])

Example fix

// before
result = SelectionSort().sort(data)  # data may be None

// after
result = SelectionSort().sort(data or [])

if data is None:
    data = []
result = SelectionSort().sort(data)

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

try:
    result = ss.sort(data)
except TypeError as e:
    if 'cannot be None' in str(e):
        result = []
    else:
        raise

When it happens

Trigger: Calling SelectionSort().sort(None); passing an unassigned or failed-to-load list variable.

Common situations: Upstream data loaders returning None on error; optional parameters defaulting to None; notebook cells run out of order so the data variable is still None.

Related errors

• array cannot be None
• sentence cannot be None
• rows and cols cannot be None
• s or t cannot be None
• a or b cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/c04028f7ed52fc35. Report an issue: GitHub.