Building a Linked List in Python with Examples

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A linked list is a linear data structure where elements are not stored next to each other in memory. The elements in a linked list are linked using pointers or references. Linked lists are an ordered collection of objects, similar to a normal list. Linked lists stand apart from lists in how they store elements in memory. While regular lists (arrays or slices) use a contiguous memory block to store references to their data, linked lists store references (pointers) as part of each element.

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A normal list is just a pointer to the first element in the list, and a specific item can be retrieved by providing a memory offset.

A linked list is also just a pointer to the first element in the list, but memory offsets won’t do us any good. We need to examine the first element’s next pointer to see where the next item is, then we can navigate to it. From there, we can find the next item and so on down the list.

Python linked list example

Node Class

First, we’ll build a Node class. The LinkedList class we eventually build will be a list of Nodes.

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

    def set_next(self, node):
        self.next = node

    def  __repr__ (self):
        return self.val

Each node has a val data member (the information it stores) and a next data member. The next data member just points to the next Node in the list if there is one, otherwise it’s None

Linked List Class Constructor

class LinkedList:
    def  __init__ (self):
        self.head = None

The constructor is easy – just initialize an empty head pointer. This indicates we now have an empty list.

Iterating over the list

Let’s make it easy to iterate over each item in the list using python’s for _ in _ syntax.

def  __iter__ (self):
        node = self.head
        while node is not None:
            yield node
            node = node.next

By implementing Python’s __iter__ method, we can now use iteration syntax. For example, for item in linked_list:.

Adding to the list

Let’s create a way to add items to the tail of the list, the add_to_tail method. It takes a node as input, iterates over the entire list, then adds the given node to the end.

def add_to_tail(self, node):
        if self.head == None:
            self.head = node
            return
        for current_node in self:
            pass
        current_node.set_next(node)

Removing from the list

There are other ways to remove items from the list, but for now, and as an example, let’s write a remove from head method.

def remove_from_head(self):
        if self.head == None:
            return None
        temp = self.head
        self.head = self.head.next
        return temp

remove_from_head removes and returns the first item from the list, assuming one exists.

Printing the linked list

Last but not least, we can implement Python’s __repr__ () method so that we can call print() directly on a list and control what it printed. Here’s a representation I like:

def  __repr__ (self):
        nodes = []
        for node in self:
            nodes.append(node.val)
        return " -> ".join(nodes)

This method will print each node’s value in order, with arrows in between. For example, hello -> this -> is -> my -> list.

Usage

linked_list = LinkedList()
linked_list.add_to_tail(Node('john'))
linked_list.add_to_tail(Node('sally'))
linked_list.add_to_tail(Node('jimmy'))
print("ll:", linked_list)
first = linked_list.remove_from_head()
print("removed:", first)
print("ll:", linked_list)

Full Linked List Code Example

class LinkedList:
    def  __init__ (self):
        self.head = None

    def  __iter__ (self):
        node = self.head
        while node is not None:
            yield node
            node = node.next

    def  __repr__ (self):
        nodes = []
        for node in self:
            nodes.append(node.val)
        return " -> ".join(nodes)

    def add_to_tail(self, node):
        if self.head == None:
            self.head = node
            return
        for current_node in self:
            pass
        current_node.set_next(node)

    def remove_from_head(self):
        if self.head == None:
            return None
        temp = self.head
        self.head = self.head.next
        return temp

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

    def set_next(self, node):
        self.next = node

    def  __repr__ (self):
        return self.val

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Comments

[Rishit Khandelwal]

You don't make linked lists in python, because you never learn pointers and memory allocation.