How to Use List Comprehension in Python

Python List Comprehension

Have you ever come across a bunch of data where you need to filter out some elements or perform certain operations on each item? You might even want to create a whole new list based on the original one. If yes, then you might have wondered if there is a single term that will make it possible to do all of this. Well, we will be learning about just that- Python List Comprehension!

In this Python Tutorial, we will try to understand Python List Comprehension and various uses of List Comprehension in Python. If you are new to Python Programming, you should first go through all the basic concepts. Python Certification Training would be the best place to get you started with Python Development.

What is Python List Comprehension?

List Comprehension in Python is a way through which we can write concise, readable codes while we are creating lists in our Python program. Traditionally, you might be using loops and conditionals and even writing several lines of code to achieve the same result. It's time for you to stop and get the easy way with list comprehension that will let you do it all in just one line. Sounds simple right?

Let's look at the basic structure of using list comprehension in a Python program.

Syntax of List Comprehension in Python

new_list = [expression for item in original_list if condition]

• expression- this will hold what has to be done with each item in the original list.
• item- this is the variable that holds each element in the original list.
• original_list-this is the list you are iterating over.
• condition- it is an optional condition that will help filter elements.

Example of List Comprehension in Python

# Original list
numbers = [1, 2, 3, 4, 5]

# List comprehension to double each number
doubled_numbers = [num * 2 for num in numbers]

print(doubled_numbers)

Output

[2, 4, 6, 8, 10]

for Loop vs. List Comprehension

Suppose, we have a list of numbers and we want to create a new list that will display only the even numbers. We will try to use both for loop and list comprehension to understand the difference.

Example using Traditional 'for' Loop:

# Original list
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# Empty list to store even numbers
even_numbers = []

# Loop to filter even numbers
for num in numbers:
    if num % 2 == 0:
        even_numbers.append(num)

print(even_numbers)

Output

[2, 4, 6, 8, 10]

Example using List Comprehension:

# Original list
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

# List comprehension to filter even numbers
even_numbers = [num for num in numbers if num % 2 == 0]

print(even_numbers)

Output

[2, 4, 6, 8, 10]

# Original list
words = ["apple", "banana", "orange", "kiwi", "pear", "strawberry"]

# List comprehension to filter words with more than 5 characters
long_words = [word for word in words if len(word) > 5]

print(long_words)

['banana', 'orange', 'strawberry']

# Original list
numbers = [1, 2, 3, 4, 5]

# List comprehension to label even and odd numbers
even_or_odd = ['Even' if num % 2 == 0 else 'Odd' for num in numbers]

print(even_or_odd)

['Odd', 'Even', 'Odd', 'Even', 'Odd']

# Nested list of numbers
matrix = [[1, 2, 3],
          [4, 5, 6],
          [7, 8, 9]]

# Nested list comprehension to transpose the matrix
transposed_matrix = [[row[i] for row in matrix] for i in range(len(matrix[0]))]

print(transposed_matrix)

[[1, 4, 7], [2, 5, 8], [3, 6, 9]]