donnemartin/interactive-coding-challenges · error · TypeError

array cannot be None

Error message

array cannot be None

What it means

Raised by Solution.merge_ranges when the array argument is None; iterating/sorting None would crash with a less clear TypeError, so the method fails fast with an explicit message. None is treated as a programming/caller error, distinct from an empty list which returns [].

Source

Thrown at online_judges/merge_ranges/merge_ranges_solution.ipynb:140

  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Solution(object):\n",
    "\n",
    "    def merge_ranges(self, array):\n",
    "        if array is None:\n",
    "            raise TypeError('array cannot be None')\n",
    "        if not array:\n",
    "            return array\n",
    "        sorted_array = sorted(array)\n",
    "        merged_array = [sorted_array[0]]\n",
    "        for index, item in enumerate(sorted_array):\n",
    "            if index == 0:\n",
    "                continue\n",
    "            start_prev, end_prev = merged_array[-1]\n",
    "            start_curr, end_curr = item\n",
    "            if end_prev < start_curr:\n",
    "                # No overlap, add the entry\n",
    "                merged_array.append(item)\n",
    "            else:\n",
    "                # Overlap, update the previous entry's end value\n",
    "                merged_array[-1] = (start_prev, max(end_prev, end_curr))\n",
    "        return merged_array"
   ]
  },

View on GitHub (pinned to 358f2cc604)

Solutions

1. Initialize the argument to [] instead of None when data may be empty.
2. Guard at the call site: merge_ranges(ranges or []).
3. Catch TypeError if None can legitimately flow through and handle it.

Example fix

# before
merged = Solution().merge_ranges(meetings)  # meetings may be None

# after
merged = Solution().merge_ranges(meetings or [])

ranges = ranges if ranges is not None else []
merged = Solution().merge_ranges(ranges)

def is_range_list(x):
    return isinstance(x, list) and all(isinstance(r, (list, tuple)) and len(r) == 2 for r in x)

try:
    merged = Solution().merge_ranges(ranges)
except TypeError as e:
    if 'cannot be None' in str(e):
        merged = []
    else:
        raise

When it happens

Trigger: Calling merge_ranges(None) or passing a variable that was initialized to None and never assigned (e.g. a failed fetch of meeting ranges).

Common situations: Optional upstream data (CSV column, API response field) that is None when missing, or a default parameter of None used as a sentinel.

Related errors

• nums cannot be None
• array cannot be None
• array cannot be None
• ransom_note or magazine cannot be None
• sentence cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/dbac563f3e85a6c6. Report an issue: GitHub.