Implementing Iter for doubly-linked list

Problem: Being new to python I am currently trying to learn the ropes, getting a better handle on the difference between an array and a linked structure. I've attempted creating a linked list class to assist in getting a better understanding of the language and its structures. What I have written so far:

class LinkedList:
    class Node:
        def __init__(self, val, prior=None, next=None):
            self.val = val
            self.prior = prior
            self.next  = next

    def __init__(self):
        self.head = LinkedList.Node(None) 
        self.head.prior = self.head.next = self.head 
        self.length = 0

    def __str__(self):
        if len(self)==0:
            return '[]'
        else:
            return '[' +  ', '.join(str(x) for x in self) + ']'

    def __repr__(self):
        """Supports REPL inspection. (Same behavior as `str`.)"""
        return str(self)

    def __len__(self):
        """Implements `len(self)`"""
        return self.length

    def __iter__(self):
        """Supports iteration (via `iter(self)`)"""
        cursor = self.head
        while cursor:
            yield cursor.val
            cursor = cursor.next

    def append(self, value):
        n = LinkedList.Node(value, prior=self.head.prior, next=self.head)
        n.prior.next = n.next.prior = n
        self.length += 1

Testing the code below, I'll have a problem where the kernel task won't end, or the debugger will point to line 7 of the test code where it failed. I don't think my __repr__ method is wrong which is why I ask, how could I edit the __iter__ method body to fix this issue? I thought all I had to do was loop through the values in cursor.

from unittest import TestCase
tc = TestCase()

lst = LinkedList()
for d in (10, 20, 30, 40, 50):
    lst.append(d)
tc.assertEqual('[10, 20, 30, 40, 50]', str(lst))
tc.assertEqual('[10, 20, 30, 40, 50]', repr(lst))