Doubly-Linked List

doubly_linked_list.py

class Node: 
    def __init__(self, key, next=None, prev=None):
        self.key = key
        self.next = next
        self.prev = prev

class DoublyLinkedList:
    def __init__(self, head=None):
        self.head = None
        self.tail = None
        
    def is_empty(self):
        return self.head == None
    
    def push_back(self, key):
        node = Node(key) 
        # set next as None
        if self.tail == None:
            self.head = node
            self.tail = node 
            # set prev as None
        else: 
            self.tail.next = node
            node.prev = self.tail
            self.tail = node
    
    def pop_back(self):
        if self.head == None:
            raise Exception("Empty List")
        if self.head == self.tail:
            self.head = None
            self.tail = None
        else:
            self.tail = self.tail.prev
            self.tail.next = None
    
    def add_after(self, node, key):
        new_node = Node(key)
        new_node.next = node.next
        new_node.prev = node
        node.next = new_node
        if new_node.next:
            new_node.next.prev = new_node
        if self.tail == node:
            self.tail = new_node
    
    def add_before(self, node, key):
        new_node = Node(key)
        new_node.next = node
        new_node.prev = node.prev
        node.prev = new_node
        if new_node.prev:
            new_node.prev.next = new_node
        if self.head == node:
            self.head = new_node

    def __str__(self):
        lst = "["
        node = self.head
        while node:
            lst += str(node.key) 
            if node.next:
                lst += ", "
            node = node.next
        return lst + "]"
            
    
l = DoublyLinkedList()
for i in range(5): l.push_back(i)
print(l)