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Python List Comprehension
Last Updated : April 30, 2025
List Comprehension
List comprehension is an important feature of Python, it provides shorter syntax to create a new list based on the existing list. List comprehension reduces the number of lines in list creation to a single line.
Syntax
The list comprehension starts with a [ and ], to ensure that the result is a list.
[expression for item in list]
The comprehension can also utilize the if condition.
Grouping elements using list comprehension
Group the common elements to a list: There are two lists with some elements, we have to create a list containing common elements.
Example
In the following example, we are finding the common elements between two lists using a list comprehension and printing the result:
# Create two lists
grade_k = ['maddy', 'sriansh', 'owen', 'molly']
grade_first = ['molly', 'owen', 'ricky', 'sid']
# Print original lists
print("Original lists")
print("grade_k:", grade_k)
print("grade_first:", grade_first)
# Grouping common elements
common_names = [a for a in grade_k for b in grade_first if a == b]
# Print new list
print("Result (common_names):", common_names)
The output of the above code will be:
Original lists
grade_k: ['maddy', 'sriansh', 'owen', 'molly']
grade_first: ['molly', 'owen', 'ricky', 'sid']
Result (common_names): ['owen', 'molly']
List of EVENs using list comprehension
List of EVEN numbers using list comprehension: Here, we will write a Python program that will create a list of EVEN numbers in a range.
Example
In the following example, we are creating a list of even numbers from 0 to 29 using list comprehension and printing the resulting list:
# List of EVEN numbers using
# list comprehension
even_list = [n for n in range(30) if n % 2 == 0]
# Print list
print(even_list)
The output of the above code will be:
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28]
List by replacing specific elements using list comprehension
Here, we will write a program to create a new list by replacing all specific elements of an existing list.
Example
In the following example, we are creating a new list by replacing the element 'molly' with 'alex' using list comprehension and printing both the original and the new lists:
# list 1
list1 = ['molly', 'sriansh', 'owen', 'molly']
# print list 1
print("list1:", list1)
# New list by replacing 'molly' with 'alex
newlist = [v if v != 'molly' else 'alex' for v in list1]
# print new list
print("newlist:", newlist)
The output of the above code will be:
list1: ['molly', 'sriansh', 'owen', 'molly']
newlist: ['alex', 'sriansh', 'owen', 'alex']
Nested List Comprehensions
Nested list comprehensions allow you to use list comprehension within the list comprehension.
Example
In the following example, we are creating a 2x4 matrix using nested list comprehension and printing the resulting matrix:
# Create a matrix using
# nested list comprehension
mat = [[j for j in range(4)] for i in range(2)]
# Print list akka matrix
print(mat)
The output of the above code will be:
[[0, 1, 2, 3], [0, 1, 2, 3]]
Advantages of using comprehension
- Comprehensions requires less code. Python interpreter is optimized to run comprehensions as quickly as possible.
- Comprehensions execute faster than for loop.
- Comprehensions can be used in places where for loop cannot be used. All the comprehensions appear to the right of the assignment operator, which is something for loop cannot do.
Python List Comprehension Exercise
Select the correct option to complete each statement about Python list comprehensions.
- A list comprehension is a concise way to create a list using a ___ expression.
- Which of the following is the correct syntax for a basic list comprehension to square each element in a list
nums = [1, 2, 3, 4]?
- Which of the following adds a condition to the list comprehension to include only even numbers from the list
nums = [1, 2, 3, 4, 5, 6]?
- What will the following list comprehension
result = [x**2 for x in range(5)] return?
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