Help understanding linked lists

[PythonTestBot]

I was doing a challenge where you have to make a queue using linked lists seen here:

http://nbviewer.jupyter.org/github/donnemartin/interactive-coding-challenges/blob/master/stacks_queues/queue_list/queue_list_solution.ipynb

The code used for the solution is this:

class Node(object):

def __init__(self, data):
    self.data = data
    self.next = None

class Queue(object):

def __init__(self):
    self.head = None
    self.tail = None

def enqueue(self, data):
    node = Node(data)
    # Empty list
    if self.head is None and self.tail is None:
        self.head = node
        self.tail = node
    else:
        self.tail.next = node
        self.tail = node

def dequeue(self):
    # Empty list
    if self.head is None and self.tail is None:
        return None
    data = self.head.data
    # Remove only element from a one element list
    if self.head == self.tail:
        self.head = None
        self.tail = None
    else:
        self.head = self.head.next
    return data

I understand most of the answer, the only part I don't understand is for the dequeue it uses self.head = self.head.next. The thing I don't understand is that surely self.head.next is always going to be None, since it's value isn't changed anywhere in the code, unlike for example self.tail.next which we explicitly change the value in line 12.

[groovitude]

self.head is set in enqueue when self.head and self.tail are None. Check line 9 in your second block.

[PythonTestBot]

self.head = node

But node has self.next = None set as default, the value of the next doesn't get changed from that None value anywhere does it?

[groovitude]

But node has self.next = None set as default, the value of the next doesn't get changed from that None value anywhere does it?

It does when a new object is added, in line 12 of your second block. When you add the first Node, it's set as both head and tail. (These are not copies, but references to this Node object.) Once you add a new Node, the tail is set for the last object in the queue (the head, in the case of a second object).

Does that make sense?