I’m working on a method to merge two sorted linked lists in Python. I encountered an issue when severing the link between nodes. Here’s my code:

class ListNode:
    def __init__(self, val=0, next=None):
        self.val = val
        self.next = next

    def __str__(self):
        values = []
        current = self
        while current:
            values.append(current.val)
            current = current.next
        return str(values)
    
    def __repr__(self) -> str:
        return str(self)

class Solution:
    def mergeTwoLists(self, list1: ListNode, list2: ListNode):
        self.res = ListNode(0)
        self.tail = self.res

        self.list1 = list1
        self.list2 = list2

        if list1 and list2:
            if list1.val <= list2.val:
                node = list1
                self.list1 = self.list1.next
                node.next = None
            else:
                node = list2
                self.list2 = self.list2.next
                node.next = None

            self.tail.next = node
            self.tail = node

        if self.list1:
            self.tail.next = self.list1
        if self.list2:
            self.tail.next = self.list2

        return self.res.next

list1 is initially [1, 2, 3]
list2 is initially [1, 3, 4]

because 1 <= 1, the first condition if list1.val <= list2.val is evaluated

after self.list1 = self.list1.next, self.list1 points to [2, 3].
However, when I set node.next = None, it seems to affect self.list1, making it [1].

Why does node.next = None affect self.list1 even though self.list1 has been reinitialized? How can I fix this issue?