input: Write a function to create a linked list in Python using a class named `ListNode`. The class should have an initializer that takes `data` as an input and sets `next` to `None`.
  
  class ListNode:
      def __init__(self, data):
          self.data = data
          self.next = None
  
  assert ListNode(5).data == 5
  assert ListNode(5).next == None
output: def create_linked_list(data_list):
      if not data_list:
          return None
      head = ListNode(data_list[0])
      current = head
      for data in data_list[1:]:
          current.next = ListNode(data)
          current = current.next
      return head