test.py

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

class Solution:

    def addTwoNumbers(self, l1, l2) -> Optional[ListNode]:
        return self.get_sum_linked_list(l1, l2)

    def split_sum_to_nodes(self, nodes_sum):
        if nodes_sum < 10:
            return ListNode(nodes_sum), 0
        else:
            return ListNode(nodes_sum % 10), 1

    def get_nodes_sum(self, node_1, node_2, carried_over: int):
        if node_1 and node_2:
            carried_over += node_1.val + node_2.val
        elif node_1:
            carried_over += node_1.val
        elif node_2:
            carried_over += node_2.val
        return carried_over

    def get_sum_linked_list(self, node_1, node_2, carry_over=0):
        next_1 = node_1.next if node_1 else None
        next_2 = node_2.next if node_2 else None
        node_from_sum_split = None
        nodes_sum = self.get_nodes_sum(node_1, node_2, carry_over)
        if nodes_sum is not None:
            node_from_sum_split, carry_over = self.split_sum_to_nodes(nodes_sum)
        elif carry_over > 0:
            # no nodes left checking carry over
            node_from_sum_split = ListNode(carry_over)
        if not next_1 and not next_2 and carry_over == 0:
            return node_from_sum_split
        else:
            node_from_sum_split.next = self.get_sum_linked_list(next_1, next_2, carry_over)
            return node_from_sum_split