In this article, we will learn how to prepend items in a list in Python. Python has an in-built method called append() to append (add at the end) items to a list.
However, there are no such in-built methods or functions to prepend (add at the beginning) items to a list in Python.
So let’s see what different methods we can use to prepend values in a Python list.
Prepend List in Python
To prepend list in Python we can use any of the following methods:
- Using + operator
- Using the insert() method.
- Using deque() function
Let’s see each of the methods with an example.
Using + operator to prepend an item to a Python List
The + operator is an arithmetic operator that is used to add values (operands) together in python.
This + operator can also be used to add two or more lists together into a single one. It merges the lists in the order we place them.
We can use the + operator to prepend multiple items to a Python list easily.
Let’s see an example below where we will prepend a list item with another one using the + operator.
Example:
color_list = ['red','green','yellow']
item = ['blue']
new_list = item + color_list
print(new_list)Output:
['blue', 'red', 'green', 'yellow']Here, we have used the + operator to add the item variable value at the beginning of the python list i.e color_list .
Prepend list using the insert() method in Python
The easy and simple way to prepend an item in a Python list is to use the insert() method.
The insert() method is used to insert values at a specific position in python.
Syntax:
list.insert(position,value)position – The specific position (index) we want the item to be inserted.
value – The item we want to insert in our python list.
So, to add an item in front of the Python list, we will use the insert() method and pass on the position and the item as parameters.
my_list = ['red','green','yellow']
my_list.insert(0, 'blue')
print(my_list)Output:
['blue', 'red', 'green', 'yellow']Here, we have inserted the item ‘blue’ in the index position 0 in our Python list.
Prepend list in Python using deque function
Deque stands for Doubly Ended Queue. It is a queue in which we can insert and delete items from both the left and right ends of the queue.
It can be implemented on any list using the collections module.
A list in python only allows us to add items at the end of the list using the append() function. A Python list cannot make space at the front to add items.
However, a deque allows us to add and remove items from the end and the front of a list.
So, to prepend a list in Python using the deque function:
- Convert the Python list to deque using the
deque()function - Using the
appendleft()method add the item at the beginning. - Convert it back to a Python list using
list()function.
Let’s see an example below.
from collections import deque
color_list = ['red','green','yellow']
deque_list = deque(color_list)
deque_list.appendleft('blue')
print(list(deque_list))Output:
['blue', 'red', 'green', 'yellow']Here, we have converted the python list into a deque object and then we have to prepend the value “blue” to the list.
Conclusion:
Here, we have learned ways to prepend one or multiple values in a Python list using different methods. The simple and easy way is the use the insert method. However, if you want faster performance then convert the list into a deque object using the deque function.
Related Article:
Get the Index or Position of Item in List in Python
How to split a list into multiple list using python
Remove Duplicates from List in Python
Python – Insert an element at specific index in a list
