Prepend to a List in Python

Prepend to a List in Python

Syed Moiz Haider Apr-29, 2021 Nov-22, 2020 Python Python List
  1. Use the insert() Method to Prepend to a List in Python
  2. Use the deque.appendleft() Method to Prepend to a List in Python
  3. Create a New List to Prepend to a List in Python

This tutorial describes how to prepend to a list in Python. The tutorial also lists some example codes to explain multiple ways of prepending to a list.

Use the insert() Method to Prepend to a List in Python

Using insert() is one of the prevalent and most used approaches. insert() is provided by the list library. The list.insert(pos, element) takes two arguments, pos and element as its parameters. pos defines the position of the element.

An example code to use this method is shown below:

lists =['james', 'tim', 'jin'] 
lists.insert (0, "steve") 

print (lists)

Output:

['steve', 'james', 'tim', 'jin']

However, the time consumed by the list.insert() operation is a little more. To improve the time performance, we can use the collections.deque method.

Use the deque.appendleft() Method to Prepend to a List in Python

The collections module of Python offers multiple data structures. In Python 2.4, deque(), a double-ended queue, is added to the collections. It is a list like a container that is efficient in appending and pop up processes. The deque data structure has an appendleft(element) method. It takes an element and appends it at the beginning of the list.

An example code for this method is given below:

import collections

dequeue = collections.deque([5,2,6,8,1])
print(dequeue)

dequeue.appendleft(10)
print(dequeue)

Output:

deque([5, 2, 6, 8, 1])
deque([10, 5, 2, 6, 8, 1])

Create a New List to Prepend to a List in Python

A very simple and trivial solution can be creating a new list with the desired element, x, for example, at the 0th index of the list. Of course, you will not prepend x to the list but create a new list with x already at the first position in the list.

A basis code for this approach is given below.

lists =['james', 'tim', 'jin'] 
new_list = ['x'] + lists
print (new_list)

Output:

['x', 'james', 'tim', 'jin']

Use List Slicing to Prepend to a List in Python

List slicing is yet another method to prepend to a list. An element is prepended to the list by assigning the 0th slice to it.

An example code for this method is as follows:

temp_list = [4,5,8,10,13]

print(temp_list)
temp_list[:0] = [12]

print(temp_list)

Output:

[4, 5, 8, 10, 13]
[12, 4, 5, 8, 10, 13]

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