geekcomputers/Python · error · Exception

Invalid Position

Error message

Invalid Position

What it means

Raised by remove_at on a singly linked list when idx is negative or idx >= length (condition `idx < 0 or self.len() <= idx`). The list cannot remove a node that does not exist, so out-of-range indices are rejected before traversal begins.

Source

Thrown at LinkedLists all Types/singly_linked_list.py:92

        while temp:
            print(f"{temp.data} ->", end=" ")
            temp = temp.next
        print("NULL")

    def len(self):
        return self.length  # O(1) length calculation
        # if self.head is None:
        #     return 0
        # count = 0
        # temp = self.head
        # while temp:
        #     count += 1
        #     temp = temp.next
        # return count

    def remove_at(self, idx):
        if idx < 0 or self.len() <= idx:
            raise Exception("Invalid Position")
        if idx == 0:
            self.head = self.head.next
            self.length -= 1
            return
        temp = self.head
        dist = 0
        while dist != idx - 1:
            dist += 1
            temp = temp.next
        temp.next = temp.next.next
        self.length -= 1

    def insert_at(self, idx: int, data):
        if idx < 0 or self.len() < idx:
            raise Exception("Invalid Position")
        if idx == 0:
            self.insert_front(data)
            return

View on GitHub (pinned to 40f4cd2652)

Solutions

  1. Use 0 <= idx < sll.len() (remember the max valid index is len()-1)
  2. To remove the last node, call remove_at(sll.len() - 1)
  3. When removing while iterating, adjust the index after each removal or iterate backwards
  4. Track the length once and re-validate after mutations

Example fix

// before
sll.remove_at(sll.len())  # Exception: Invalid Position

// after
if sll.len() > 0:
    sll.remove_at(sll.len() - 1)  # removes tail
Defensive patterns

Strategy: validation

Validate before calling

if 0 <= idx < sll.len():
    sll.remove_at(idx)
# to pop tail:
if sll.len():
    sll.remove_at(sll.len() - 1)

Type guard

def is_valid_remove_idx(sll, idx) -> bool:
    return isinstance(idx, int) and 0 <= idx < sll.len()

Try / catch

try:
    sll.remove_at(idx)
except Exception as e:
    if str(e) == 'Invalid Position':
        pass  # already removed / nothing there
    else:
        raise

Prevention

When it happens

Trigger: Calling sll.remove_at(idx) with idx < 0 or idx >= sll.len(), e.g. remove_at(len()) to pop the last element (the last valid index is len()-1).

Common situations: Pop-from-end style code assuming remove_at(len()) removes the tail; loop counters off by one; stale indices after other removals in the same loop.

Related errors

AI-assisted analysis of geekcomputers/Python@40f4cd2652 (2026-08-27). Data as JSON: /api/errors/a517f7cba7cf2f99. Report an issue: GitHub.