[Python] Why does unpacking a 3-element tuple from heapq.heappop fail when using _, _, node =...

I'm working on Merge K Sorted Linked Lists from Leetcode and the optimal solution requires use of a heap. When unpacking the tuple from heapq.heappop(), I figured I would use _, _, node to unpack, as I'm not using the value or index anywhere else in the code. However this throws the following TypeError:



I understand that this error is the result of the heap trying to compare the ListNode object instead of the values and index, but this should not be the case as I've clearly defined it to compare the first 2 objects before the last. When unpacking with val, i, node = heapq.heappop(heap) the code runs fine.

import heapq
# Definition for singly-linked list.
# class ListNode:
# def __init__(self, val=0, next=None):
# self.val = val
# self.next = next

class Solution:
def mergeKLists(self, lists: List[Optional[ListNode]]) -> Optional[ListNode]:
heap = []

dummy = ListNode()
curr = dummy

for i, node in enumerate(lists):
if node:
heapq.heappush(heap, (node.val, i, node)) #Add to the heap. Compare by value, then index

while heap:
val, i, node = heapq.heappop(heap) #Working line
#_, _, node = heapq.heappop(heap) #Non-working line
curr.next = node
curr = node
node = node.next
if node: #If we're not at the end of the list, add to the heap
heapq.heappush(heap, (node.val, i, node))

return dummy.next

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