title: Python : AttributeError: 'NoneType' object has no attribute 'data'
body: <p>I am trying to merge 2 sorted linked list into single sorted linked list.</p>
  
  <pre><code>class Node:
      # Function to initialise the node object
      def __init__(self, data):
          self.data =data
          self.next = None
  
  class LinkedList:
         def mergeNode(self, second_list):
          llist4 = LinkedList()
          first_node = self.head
          second_node = second_list.head
  
          while (first_node!=None or second_node != None):
              if (first_node.data &gt; second_node.data):
  
                      llist4.push(first_node)
                      prev = first_node
                      first_node = first_node.next
  
              elif (first_node.data &lt; second_node.data):
  
                      llist4.push(second_node)
                      prev = second_node
                      second_node = second_node.next
  
          else:
                  llist4.push(first_node)
                  llist4.push(second_node)
                  prev = second_node
                  second_node = second_node.next
                  first_node = first_node.next
          return llist4
  
  if __name__ == '__main__':
  
      llist = LinkedList()
      llist2 = LinkedList()
      llist3 = LinkedList()
  
      llist.push(15)
      llist.push(10)
      llist.push(5)
  
      llist2.push(12)
      llist2.push(3)
      llist2.push(2)
          llist3 = llist.mergeNode(llist2)
      llist3.printList()
  </code></pre>
  
  <p>But I am getting error as :<br>
  <code>" AttributeError: 'NoneType' object has no attribute 'data'"</code><br>
   I am aware that I might be using None value in my code by I am not able to figure out the reason. Someone please help me to find out the problem?</p>