donnemartin/interactive-coding-challenges · error · TypeError

key cannot be None

Error message

key cannot be None

What it means

Raised by MinHeap.insert when key is None. Heap ordering comparisons (via _bubble_up) would raise TypeError against None anyway; this guard gives a clearer, earlier error. None keys are fundamentally unorderable in a min-heap.

Source

Thrown at graphs_trees/min_heap/min_heap.py:30

        return len(self.array)

    def extract_min(self):
        if not self.array:
            return None
        if len(self.array) == 1:
            return self.array.pop(0)
        minimum = self.array[0]
        # Move the last element to the root
        self.array[0] = self.array.pop(-1)
        self._bubble_down(index=0)
        return minimum

    def peek_min(self):
        return self.array[0] if self.array else None

    def insert(self, key):
        if key is None:
            raise TypeError('key cannot be None')
        self.array.append(key)
        self._bubble_up(index=len(self.array) - 1)

    def _bubble_up(self, index):
        if index == 0:
            return
        index_parent = (index - 1) // 2
        if self.array[index] < self.array[index_parent]:
            # Swap the indices and recurse
            self.array[index], self.array[index_parent] = \
                self.array[index_parent], self.array[index]
            self._bubble_up(index_parent)

    def _bubble_down(self, index):
        min_child_index = self._find_smaller_child(index)
        if min_child_index == -1:
            return
        if self.array[index] > self.array[min_child_index]:

View on GitHub (pinned to 358f2cc604)

Solutions

1. Filter None keys before insertion or substitute a large sentinel priority
2. Fix the producer so priorities are always set (validate at ingest)
3. If None means 'lowest priority', map it explicitly (e.g. float('inf'))

Example fix

# before
heap.insert(item.get('priority'))

# after
priority = item.get('priority')
heap.insert(priority if priority is not None else float('inf'))

if key is not None:
    heap.insert(key)

def is_heap_key(value) -> bool:
    return value is not None and isinstance(value, (int, float))

try:
    heap.insert(key)
except TypeError:
    heap.insert(float('inf'))  # or skip

When it happens

Trigger: Calling heap.insert(None); inserting values from a stream that contains None (e.g. API responses with optional fields).

Common situations: Priority queues fed by data with missing priorities; mixing Optional values into heap items after a refactor.

Related errors

• data cannot be None
• neighbor or weight cannot be None
• neighbor cannot be None
• key cannot be None
• word cannot be None

AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28). Data as JSON: /api/errors/fc9bc1eb7ca5d0b9. Report an issue: GitHub.