TheAlgorithms/Python · error · IndexError

list index out of range

Error message

list index out of range

What it means

LinkedList.insert_nth raises IndexError('list index out of range') when index is outside [0, len]. Index == len is the legal append position; negative indices always fail. This is the IndexError counterpart to the ValueError used by __getitem__/__setitem__ in the same file — the class mixes exception types by operation.

Source

Thrown at data_structures/linked_list/singly_linked_list.py:209

    def insert_nth(self, index: int, data: Any) -> None:
        """
        Insert data at given index.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("first")
        >>> linked_list.insert_tail("second")
        >>> linked_list.insert_tail("third")
        >>> linked_list
        first -> second -> third
        >>> linked_list.insert_nth(1, "fourth")
        >>> linked_list
        first -> fourth -> second -> third
        >>> linked_list.insert_nth(3, "fifth")
        >>> linked_list
        first -> fourth -> second -> fifth -> third
        """
        if not 0 <= index <= len(self):
            raise IndexError("list index out of range")
        new_node = Node(data)
        if self.head is None:
            self.head = new_node
        elif index == 0:
            new_node.next_node = self.head  # link new_node to head
            self.head = new_node
        else:
            temp = self.head
            for _ in range(index - 1):
                temp = temp.next_node
            new_node.next_node = temp.next_node
            temp.next_node = new_node

    def print_list(self) -> None:  # print every node data
        """
        This method prints every node data.
        >>> linked_list = LinkedList()
        >>> linked_list.insert_tail("first")

View on GitHub (pinned to f5988cc097)

Solutions

1. Validate 0 <= index <= len(linked_list) before calling.
2. Prefer insert_tail()/insert_head() for the two common ends rather than computing indices.
3. Catch IndexError for insert failures, ValueError for get/set failures — do not share one handler blindly.

Example fix

# before
linked_list.insert_nth(len(linked_list) + 1, 'x')  # IndexError
# after
linked_list.insert_nth(len(linked_list), 'x')  # legal append

def safe_insert_nth(ll, index, data):
    if 0 <= index <= len(ll):
        ll.insert_nth(index, data)
        return True
    return False

try:
    ll.insert_nth(i, data)
except IndexError:
    ll.insert_tail(data)  # fallback: append

When it happens

Trigger: insert_nth(5, x) on a 4-node list; insert_nth(-1, x); index arithmetic off by one when inserting before/after a located node.

Common situations: Insertion loops with range(len(linked_list)+2); mixing up the inclusive/exclusive bounds between insert (<= len) and delete (< len); 1-based positions not converted.

Related errors

• list index out of range
• list index out of range.
• List index out of range.
• Position must be non-negative
• Out of bounds

AI-assisted analysis of TheAlgorithms/Python@f5988cc097 (2026-08-14). Data as JSON: /api/errors/6d497f6dea8dccda. Report an issue: GitHub.