I am getting this error. I go through this concept but could not find the error. I am making a link list but when giving values it's showing me this error TypeError: DoubleLinkedList.add() takes 1 positional argument but 2 were givenI got through the below concept but I could not find the error or what to do. I am new in python programming

        class Node:
            def __init__(self, value):
                self.next = None
                self.prev = None
                self.val = value
        
        
        class DoubleLinkedList:
            def __init__(self, ):
                self.head = None
                self.tail = None
                self.size = 0
        
            def add(self):
                node = Node()
                if self.tail is None:
                    self.head = node
                    self.tail = node
                    self.size  = 1
                else:
                    self.tail.next = node
                    node.prev = self.head
                    self.tail = node
                    self.size  = 1
        
            def __str__(self):
                vals = []
                node = self.head
                while node is not Node:
                    vals.append(node.val)
                    node = node.next
                return f"[{','.join((str(val) for val in vals))}]"
        
        
        my_list = DoubleLinkedList()
        
        my_list.add(1)
        my_list.add(5)
        my_list.add(2)
        
        print(my_list)
    
    ```

CodePudding user response：

The reason why you see this error is because you only let add(self) to have one positional argument, which is self. What you want is something like follows:

def add(self, value):
    node = Node(value)
    if self.tail is None:
        self.head = node
        self.tail = node
        self.size  = 1
    else:
        self.tail.next = node
        node.prev = self.head
        self.tail = node
        self.size  = 1

Note that this answer doesn't look for your logic, only to fix your problem regarding positional arguments.

Also, self is a special keyword in Python used for instance variables and methods. When you create a new instance of Node as node = Node(), you automatically pass self argument, and the same goes for add method.

When you call my_list.add(1), Python inherently understands it as

my_list.add(self,1)

Of course, here self is equal to my_list. That's why Python tells you "I already got self, however, you've passed another value, so I will throw an error.

CodePudding user response：

You have to modify your add function like this

    def add(self, value):
        node = Node(value)
        if self.tail is None:
            self.head = node
            self.tail = node
            self.size  = 1
        else:
            self.tail.next = node
            node.prev = self.head
            self.tail = node
            self.size  = 1

And you should modify a little also the __str__ function

def __str__(self):
    vals = []
    node = self.head
    while node is not None:
        vals.append(node.val)
        node = node.next
    return ','.join((str(val) for val in vals))