donnemartin/interactive-coding-challenges · error · TypeError

data cannot be None

Error message

data cannot be None

What it means

QuickSort.sort raises TypeError('data cannot be None') as an explicit guard before delegating to the recursive _sort. Without it, len(data) inside _sort would raise an AttributeError on None. Only None is rejected; an empty list is valid and returned as-is.

Source

Thrown at sorting_searching/quick_sort/quick_sort_solution.ipynb:113

   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "from __future__ import division\n",
    "\n",
    "\n",
    "class QuickSort(object):\n",
    "\n",
    "    def sort(self, data):\n",
    "        if data is None:\n",
    "            raise TypeError('data cannot be None')\n",
    "        return self._sort(data)\n",
    "\n",
    "    def _sort(self, data):\n",
    "        if len(data) < 2:\n",
    "            return data\n",
    "        equal = []\n",
    "        left = []\n",
    "        right = []\n",
    "        pivot_index = len(data) // 2\n",
    "        pivot_value = data[pivot_index]\n",
    "        # Build the left and right partitions\n",
    "        for item in data:\n",
    "            if item == pivot_value:\n",
    "                equal.append(item)\n",
    "            elif item < pivot_value:\n",
    "                left.append(item)\n",
    "            else:\n",
    "                right.append(item)\n",

View on GitHub (pinned to 358f2cc604)

Solutions

1. Pass a list (use data or [] if None means empty) before calling sort()
2. Guard the call site: if data is not None: result = qs.sort(data)
3. Fix the upstream code that produced None instead of a list

Example fix

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

// after
result = QuickSort().sort(data) if data is not None else []

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

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

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

When it happens

Trigger: Calling QuickSort().sort(None), or passing a None-valued variable (failed load, unset default parameter, conditional that never assigned a list).

Common situations: Optional config/data that defaults to None; empty results from a database query being confused with None; tests exercising invalid input.

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/54748ebc4ab3de0c. Report an issue: GitHub.