donnemartin/interactive-coding-challenges · error · TypeError

k cannot be None

Error message

k cannot be None

What it means

Raised by Solution.longest_substr when k is None. k is compared as len(chars_to_index_map) > k; None would break that comparison, so the method requires an integer k (the max number of distinct characters) explicitly.

Source

Thrown at online_judges/longest_substr_k_distinct/longest_substr_solution.ipynb:104

   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Code"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {},
   "outputs": [],
   "source": [
    "class Solution(object):\n",
    "\n",
    "    def longest_substr(self, string, k):\n",
    "        if string is None:\n",
    "            raise TypeError('string cannot be None')\n",
    "        if k is None:\n",
    "            raise TypeError('k cannot be None')\n",
    "        low_index = 0\n",
    "        max_length = 0\n",
    "        chars_to_index_map = {}\n",
    "        for index, char in enumerate(string):\n",
    "            chars_to_index_map[char] = index\n",
    "            if len(chars_to_index_map) > k:\n",
    "                low_index = min(chars_to_index_map.values())\n",
    "                del chars_to_index_map[string[low_index]]\n",
    "                low_index += 1\n",
    "            max_length = max(max_length, index - low_index + 1)\n",
    "        return max_length"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Unit Test"

View on GitHub (pinned to 358f2cc604)

Solutions

1. Pass an integer k, e.g. longest_substr('eceba', 2)
2. Default at the call site: k = 2 if k is None else k
3. Validate config/argv before the call and fail with a clear message

Example fix

# before
solution.longest_substr(s, config.get('k'))

# after
k = config.get('k')
if k is None:
    raise ValueError('k is required')
solution.longest_substr(s, int(k))

if k is None: raise ValueError('k is required')
solution.longest_substr(string, int(k))

def is_int(x): return isinstance(x, int)

try:
    n = solution.longest_substr(s, k)
except TypeError:
    n = solution.longest_substr(s, DEFAULT_K)

When it happens

Trigger: Calling longest_substr('abc', None) — usually a k read from argv/config that was never set, or an optional parameter defaulting to None.

Common situations: CLI parsing where k = args.k with no default; config-driven workloads with a missing key; tests asserting the guard.

Related errors

• string 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/2e219599b38081a0. Report an issue: GitHub.