input: Write a function that implements the `ListNode` class for creating a linked list in Python with nodes. The class should have an `__init__` method to initialize the data and the next node reference.
  
  
  class ListNode:
      def __init__(self, data):
          self.data = data
          self.next = None
  
  # Example usage
  node1 = ListNode(1)
  node2 = ListNode(2)
  node1.next = node2
  
  
  assert ListNode(1).data==1
  assert ListNode(2).next==None
output: def __init__(self, data):
      self.data = data
      self.next = None