input: Write a function to implement a linked list using classes in Python, including methods to create nodes, link the nodes, and print the linked list. Create an instance of the linked list and test it with nodes containing values 1, 2, and 3. Print the linked list after linking the nodes. 
  
  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
  
  # Printing the linked list
  linked_list.print_list()
  
  assert linked_list.head.data == 1
  assert linked_list.head.next.data == 2
  assert linked_list.head.next.next.data == 3
  assert linked_list.head.next.next.next == None
output: def __init__(self):
      self.head = None
  
  def print_list(self):
      node = self.head
      while(node):
          print(node.data)
          node = node.next