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Python – Extend deque to the left

In this tutorial, we will look at how to extend a deque to the left in Python with the help of some examples.

deque in Python

The collections module in Python comes with a deque class that you can use to implement a stack or queue data structure in Python. Deques support thread-safe, memory efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction.

In simple terms, you can think of a deque in Python as a list-like data structure but with efficient insertion and removal of items from either end (left and right). This makes them very useful when implementing a stack or queue functionality in Python.

How to append multiple elements to the left side of deque in Python?

extend deque to the left in python

The deque class comes with a number of built-in functions to help perform common operations (like adding and removing elements).

You can extend a deque to the left if you want to append multiple elements at once to the beginning (the left side) of a deque. To extend a deque in Python to the left (append multiple elements on the left at once), use the deque extendleft() function.

The following is the syntax –

# add elements from iterable to the left of deque

It appends the elements from the iterable to the left of the deque.

Example – Append multiple elements to a deque

A common use case for the deque extendleft() function is when you have to add multiple elements at once to the left side of the deque.

First, let’s create a deque with some elements.

from collections import deque

# create a deque
queue = deque([1, 2, 3])
# print the deque


deque([1, 2, 3])

Here, we imported the deque class from the collections module and created a deque object, queue with three elements – 1, 2, and 3.

Now, let’s add some elements to this deque object on its left side (the start of the deque).

# add elements 4 and 5 to the left side of the deque
queue.extendleft([4, 5])
# print the deque


deque([5, 4, 1, 2, 3])

Here, we used the extendleft() function to add the elements 4 and 5 to the start of the deque queue. We passed the elements we wanted to add on the left side inside a list (an iterable) to the extendleft() function.

The order in which the elements are present in the iterable is important as the same order will be used to add the elements to the deque. Here, notice that first 4 is added to the left side and then 5.

Note that, if you would’ve used the appendleft() function to perform the same operation, you would’ve had to call the function each time for adding each individual item separately.

Difference between extendleft() and extend()

Both the extendleft() and the extend() functions are used to add multiple elements at once to a deque in Python. In both functions, we pass the iterable containing the elements to be added.

The main difference between them is that the extend() function is used to add the elements to the end (the right side) of the deque whereas the extendleft() function is used to add the elements to the start (the left side) of the deque.

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  • Piyush

    Piyush is a data scientist passionate about using data to understand things better and make informed decisions. In the past, he's worked as a Data Scientist for ZS and holds an engineering degree from IIT Roorkee. His hobbies include watching cricket, reading, and working on side projects.