input: 
  def create_linked_list():
      """
      Implement a linked list using classes in Python. The linked list should be able to add nodes and print the list.
      
      >>> linked_list = create_linked_list()
      >>> isinstance(linked_list, LinkedList)
      True
      >>> linked_list.head.data
      1
      >>> linked_list.head.next.data
      2
      >>> linked_list.head.next.next.data
      3
      >>> linked_list.head.next.next.next is None
      True
      """
      class Node:
          """Node class to represent the node in a linked list"""
          def __init__(self, data):
              self.data = data
              self.next = None
  
      class LinkedList:
          """LinkedList class to represent the entire linked list"""
          def __init__(self):
              self.head = None
          
          def print_list(self):
              node = self.head
              while(node):
                  print(node.data)
                  node = node.next
  
      # Creating a linked list 
      linked_list = LinkedList()
  
      # Creating the nodes
      node1 = Node(1)
      node2 = Node(2)
      node3 = Node(3)
  
      # Linking the nodes
      linked_list.head = node1
      node1.next = node2
      node2.next = node3
  
      return linked_list
  
output:     def add_node(self, data):
          new_node = Node(data)
          if self.head is None:
              self.head = new_node
          else:
              last_node = self.head
              while last_node.next:
                  last_node = last_node.next
              last_node.next = new_node