donnemartin/interactive-coding-challenges · error · TypeError

k must be of type int

Error message

k must be of type int

What it means

Raised by Solution.longest_substr when k is not an int (including None). It is the second guard, after the string check, because the algorithm compares len(chars_to_index_map) > k numerically; a non-int k would make that comparison meaningless or raise elsewhere.

Source

Thrown at recursion_dynamic/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 not isinstance(string, str):\n",
    "            raise TypeError('string must be of type str')\n",
    "        if not isinstance(k, int):\n",
    "            raise TypeError('k must be of type int')\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. Convert at the boundary: k = int(k) for str/float inputs
2. Provide a concrete default: def longest_substr(string, k=1) at your own wrapper
3. Validate k is int (and >= 0) before calling

Example fix

// before
sol.longest_substr(s, k=request.args.get('k'))
// after
sol.longest_substr(s, k=int(request.args.get('k', 2)))

k = int(k) if k is not None else 1
sol.longest_substr(string, k)

def is_int(x):
    return isinstance(x, int) and not isinstance(x, bool)

try:
    sol.longest_substr(string, k)
except TypeError:
    k = int(k)
    n = sol.longest_substr(string, k)

When it happens

Trigger: Calling longest_substr('abc', None), longest_substr('abc', '2'), or longest_substr('abc', 2.0). Note bool is a subclass of int and would pass this check.

Common situations: Taking k from CLI args or query strings where it arrives as str; k read from JSON config as float; forgotten default leaving k=None.

Understand the failure class

Background: "Wrong argument type", "must be a string", "expected Array or Prism::Scope": TypeError and ArgumentError when a library receives a value of the wrong type — this error's family across 28 libraries.

Related errors

• string must be of type str
• string cannot be None
• k cannot be None
• num cannot be None
• max_num cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/2f9bc4efb46158fa. Report an issue: GitHub.