This code should create a node but I have a problem with it I tried to fix it but I couldn`t

I want to know why there is a problem at in the Linked_List (Next_Node)

that is what show in the error "(Cannot assign member "next_node" for type "node" Expression of type "node | None" cannot be assigned to member "next_node" of class "node" Type "node | None" cannot be assigned to type "None" Type cannot be assigned to type "None")"

class node :
    data = None
    next_node = None

def __init__(self , data) :
    self.data = data

def __repr__(self) :
    return "<node data: %s>" % self.data

class linked_list :

def __init__(self ):
    self.head = None

def add (self , data):
    new_node = node(data)
    new_node.next_node = self.head
    self.head = new_node 
def __repr__ (self):
    nodes =[]
    current = self.head
    while current :
        if current is self.head:
            nodes.append ("[:head %s ]" % current.data)
        elif current.next.node is None :
            nodes.append ("[tail: %s ]" % current.data)
        else :
            nodes.append ("[:%s ]" % current.data)
    current = current.next_node
    return  "->".join(nodes) 

CodePudding user response：

There are several problems with current attempt:

1. The Node class as pointed by John Gordon is wrongly constructed. The data and next_node should be in __init__ method.
2. The add method is not adding new node in correct position.
3. The __repr__ is not looping through all the nodes in the linked list because of wrong indentation.

Updated code:

class node:
    def __init__(self, data):
        self.data = data
        self.next_node = None

    def __repr__(self):
        return "<node data: %s>" % self.data


class linked_list:

    def __init__(self):
        self.head = None

    def is_empty(self):
        return self.head == None

    def size(self):
        current = self.head
        count = 0

        while current:
            count  = 1
            current = current.next_node
        return count

    def add(self, data):
        new_node = node(data)
        if self.head == None:
            self.head = new_node
        else:
            current = self.head
            while current.next_node != None:
                current = current.next_node
            current.next_node = new_node

    def __repr__(self):
        nodes = []
        current = self.head
        while current:
            if current is self.head:
                nodes.append("[:head %s ]" % current.data)
            elif current.next_node is None:
                nodes.append("[tail: %s ]" % current.data)
            else:
                nodes.append("[:%s ]" % current.data)
            current = current.next_node
        return "->".join(nodes)


l = linked_list()
l.add(1)
l.add(2)
l.add(3)
print(l)

Output:

[:head 1 ]->[:2 ]->[tail: 3 ]

CodePudding user response：

class node :
    data = None
    next_node = None

    def __init__(self , data):
        self.data = data

I think the problem is because of the node class definition.

The way you've defined the next_node variable, it is a direct attribute of the class, so it is shared among all instances of the class.

I think you intended that attribute to be inside the __init__ method, so that each instance would have its own separate copy of that variable:

class node :
    def __init__(self , data) :
        self.data = data
        self.next_node = None