Hackerrank Merged two sorted linked lists

mergelists.py

import sys


class SinglyLinkedListNode:
    def __init__(self, node_data):
        self.data = node_data
        self.next = None


class SinglyLinkedList:
    def __init__(self):
        self.head = None
        self.tail = None

    def insert_node(self, node_data):
        node = SinglyLinkedListNode(node_data)

        if not self.head:
            self.head = node
        else:
            self.tail.next = node

        self.tail = node


def print_singly_linked_list(node, sep, fptr):
    while node:
        fptr.write(str(node.data))

        node = node.next

        if node:
            fptr.write(sep)


def merge_lists(head1, head2):
    root = SinglyLinkedList()
    while head1 and head2:
        if head1.data <= head2.data:
            root.insert_node(head1.data)
            head1 = head1.next
        else:
            root.insert_node(head2.data)
            head2 = head2.next
    while head1:
        root.insert_node(head1.data)
        head1 = head1.next
    while head2:
        root.insert_node(head2.data)
        head2 = head2.next
    return root.head


if __name__ == "__main__":
    l1 = [1, 4, 7, 8, 900]
    l2 = [2, 8, 9, 11, 100]

    list1 = SinglyLinkedList()
    for i in l1:
        list1.insert_node(i)
    list2 = SinglyLinkedList()
    for j in l2:
        list2.insert_node(j)

    merged_list = merge_lists(list1.head, list2.head)
    print_singly_linked_list(merged_list, " ", sys.stdout)
    print()