List comprehension with condition in Python

Introduction

List comprehension with condition helps you create a new list by picking only the items you want from another list, making your code shorter and easier to read.

When you want to filter out unwanted items from a list.

When you want to create a list of squares of only even numbers from another list.

When you want to quickly make a list of words that start with a certain letter.

When you want to transform items but only if they meet a certain rule.

Syntax

Python

new_list = [expression for item in old_list if condition]

The 'expression' is what you want to put in the new list for each item.

The 'condition' is a test that decides if the item should be included.

Examples

Creates a list of even numbers from the original list.

Python

numbers = [1, 2, 3, 4, 5]
even_numbers = [num for num in numbers if num % 2 == 0]

Filters words that start with the letter 'a'.

Python

words = ['apple', 'banana', 'cherry', 'date']
a_words = [word for word in words if word.startswith('a')]

Works fine even if the original list is empty; result is an empty list.

Python

empty_list = []
filtered = [x for x in empty_list if x > 0]

Works with a list of one item, includes it if condition is true.

Python

single_item = [10]
filtered_single = [x for x in single_item if x > 5]

Sample Program

This program takes a list of numbers, then creates a new list with squares of only the even numbers, and prints both lists.

Python

numbers = [10, 15, 20, 25, 30]
print('Original list:', numbers)
even_squares = [num ** 2 for num in numbers if num % 2 == 0]
print('Squares of even numbers:', even_squares)

Important Notes

Time complexity is O(n), where n is the number of items in the original list.

Space complexity is O(m), where m is the number of items that meet the condition.

A common mistake is forgetting the 'if' condition, which includes all items instead of filtering.

Use list comprehension with condition when you want a quick, readable way to filter and transform lists.

Summary

List comprehension with condition lets you make a new list by choosing items that pass a test.

It makes your code shorter and easier to understand than using loops and if statements.

It works well even with empty lists or lists with one item.