Python method not affecting instance of Class

Question

My question is in the del_node method, despite setting self = self.next , this seems to have no affect on the instance of the class that's making the call.

for instance, if i have a linked list L = 1 -> 2 -> 3 -> 4

calling L.del_node(x) , where x is any node other than the first, simply making the call this way will work, however, if I were to call L.del_node(1) I have to call it as L = L.del_node(1); I'm assuming this has to do with not changing any values of the instance as I am with the others (eg resetting self.data / next , etc).

I ask because I'm confused why the statement self = self.next doesn't seem to affect the instance when self.data = ... does.

class Node():

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

    def add_node(self,data):

        if self.data == None:
            self.data = data

        else:
            while self.next != None:
                self = self.next
            self.next = Node(data)


    def del_node(self,data):

        if self.data == None:
            print "ERROR: Null list"
            return self

        elif self.data == data:
            if self.next == None:
                self.data = None
                return self
            else:
                #this seems to be the only case where
                #instance = instance.del_node() matters (why?)
                #opposed to just instance.del_node()
                return self.next

        start = self
        prev = self
        self = self.next

        while self.next != None:
            if self.data == data:
                prev.next = self.next
                return start
            prev = self
            self = self.next

        if self.data == data:
            prev.next = None
            return start
        else:
            print "ERROR: value not in list"
            return start

    def get_len(self):
        length = 0
        if self.data == None:
            return length

        while self.next != None:
            length += 1
            self = self.next
        return length + 1

    def __str__(self):

        string = ''

        while self.next != None:
            string += str(self.data) + ' -> '
            self = self.next
        return string + str(self.data)

Answer 1

self = something does not change the object. Just as everywhere else in Python, it merely rebinds the local name self . (There is nothing special to Python about the name self --it's only used by convention. A method is just a normal function and self is just a normal argument. It's how it's called that passes self .)