Receiving error when merging two linked lists

Question

I'm trying to merge two linked lists in Python, but every time I receive the error: type object 'List' has no attribute 'merge' when it clearly has the function for merging.

---

Sample Input:
8 11 20 24 50
5 9 10 30 33 40 45
Sample Output:
5 8 9 10 11 20 24 30 33 40 45 50

---

class Node: 
    def __init__ (self, data): 
       self.data = data 
       self.next = None
class List: 
    def __init__(self): 
       self.head = None
    def append(self, new_data): 
       new_node = Node(new_data) 
       if self.head is None: 
            self.head = new_node 
            return
       last = self.head 
       while last.next: 
            last = last.next
       last.next = new_node 
    def merge(head1, head2): 
       temp = None
       if head1 is None: 
          return head2 
       if head2 is None: 
          return head1 
       if head1.data <= head2.data: 
          temp = head1 
          temp.next = merge(head1.next, head2) 
       else: 
          temp = head2
          temp.next = merge(head1, head2.next)
       return temp 

For taking input:

inp = [int(i) for i in input().split(" ")]
inp2 = [int(i) for i in input().split(" ")]
l = List()
l2 = List()
for i in inp:
    l.append(i)
for i in inp2:
    l2.append(i)
rez = List.merge(l, l2)
print(rez)

Thank you for your time!

Answer 1

If you want your method to be callable on the class rather than on an object, you should make it static:

@staticmethod
def merge(head1, head2): 
    # etc...

You could also make your code a lot clearer (including to yourself) by using type hints as you are mixing up what is a List and what is a Node:

from typing import Any, Optional

class Node:
    def __init__(self, data: Any):
        self.data = data
        self.next: Optional[Node] = None  # might be Optional["Node"] since it inside it's own definition, I'm not sure check the mypy docs


class LinkedList:
    def __init__(self, head: Optional[Node] = None):
        self.head = head

    def append(self, new_data: Any) -> None:
        new_node = Node(new_data)
        if self.head is None:
            self.head = new_node
            return
        last = self.head
        while last.next:
            last = last.next
        last.next = new_node

    @staticmethod
    def merge(list1: "LinkedList", list2: "LinkedList") -> "LinkedList":
        if list1.head is None:
            return list2
        if list2.head is None:
            return list1
        if list1.head.data <= list2.head.data:
            head = list1.head
            list1 = LinkedList(list1.head.next)
        else:
            head = list2.head
            list2 = LinkedList(list2.head.next)
        temp = LinkedList(head)
        temp.head.next = LinkedList.merge(list1, list2).head
        return temp 

test it with:

inp = [8, 11, 20, 24, 50]
inp2 = [5, 9, 10, 30, 33, 40, 45]
l = LinkedList()
l2 = LinkedList()
for i in inp:
    l.append(i)
for i in inp2:
    l2.append(i)
rez = LinkedList.merge(l, l2)

temp = rez.head
print(temp.data)
while temp.next:
    temp = temp.next
    print(temp.data)

Answer 2

You're getting yourself muddled with the heads and the list object types. You're calling it with lists sometimes and recursively calling it with heads, fixed it up for ya:

class Node: 
    def __init__ (self, data): 
       self.data = data 
       self.next = None
class List: 
    def __init__(self): 
       self.head = None
    def append(self, new_data): 
       new_node = Node(new_data) 
       if self.head is None: 
            self.head = new_node 
            return
       last = self.head 
       while last.next: 
            last = last.next
       last.next = new_node 
    def merge(head1, head2):
       list_given = type(head1) == List
       if list_given:
           head1 = head1.head
           head2 = head2.head
       temp = None
       if head1 is None: 
          return head2 
       if head2 is None: 
          return head1 
       if head1.data <= head2.data: 
          temp = head1 
          temp.next = List.merge(head1.next, head2) 
       else: 
          temp = head2
          temp.next = List.merge(head1, head2.next)

       if list_given:
           new_list = List()
           new_list.head = temp
           return new_list
       else:
           return temp 


inp = [int(i) for i in input().split(" ")]
inp2 = [int(i) for i in input().split(" ")]

l = List()
l2 = List()
for i in inp:
    l.append(i)
for i in inp2:
    l2.append(i)
rez = List.merge(l, l2)
print(rez)