How to connect single linked list

I wanted to solve the palindrome linked list problem.

I went through the linked list, but I don't know how to refer to it in a different function. Here's my code (not working):

def __init__ (self):
    self.head = None

def add_end(self, data):
    newNode = Node(data)
    n = self.head
    if self.head is None:
        self.head= newNode

    n = self.head
    while n:
        n = n.ref
    return self.head

def palindrom(self):

    fast = self.head
    slow = self.head

    while fast and fast.ref:
        fast = fast.next.ref
        slow = slow.ref

    #reserve second half
    prev = None
    while slow:
        tmp = slow.ref
        slow.ref = prev
        prev =slow
        slow = tmp

Could you please help me solve the problem?