What Is List Comprehension in Python?

List comprehension is a concise way to create new lists in Python using a single line of code. It combines the logic of a for loop, and optional conditions, into a clear expression that produces a list. List comprehension makes your code shorter and often more readable than equivalent loops.

Basic List Comprehension Syntax

new_list = [expression for item in iterable]

  • expression — value or operation applied to each item

  • item — variable representing each element

  • iterable — sequence like a list, range, string, etc.

Examples (Input & Output Together)

1. Create a List from Another List

Python 
nums = [1, 2, 3, 4, 5] squares = [x * x for x in nums] print(squares) # Output: # [1, 4, 9, 16, 25]

This generates a list of squares for each number.

2. List Comprehension with a range()

Python 
evens = [x for x in range(1, 11) if x % 2 == 0] print(evens) # Output: # [2, 4, 6, 8, 10]

Here we include a condition to select only even numbers.

List Comprehension with Condition

Python 
nums = [1, 2, 3, 4, 5, 6] greater_than_three = [x for x in nums if x > 3] print(greater_than_three) # Output: # [4, 5, 6]

The list includes only values greater than 3.

Nested List Comprehension

You can use a list comprehension inside another to produce combinations

Python 
pairs = [(i, j) for i in [1, 2, 3] for j in ["a", "b"]] print(pairs) # Output: # [(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b'), (3, 'a'), (3, 'b')]

This creates every pair of (number, letter).

Conditional Expression Inside Comprehension

You can use a ternary expression to modify values conditionally

Python 
nums = [1, 2, 3, 4, 5] labels = ["even" if x % 2 == 0 else "odd" for x in nums] print(labels) # Output: # ['odd', 'even', 'odd', 'even', 'odd']

Each item is labeled based on parity.

List Comprehension with Strings

You can also create lists from strings

Python 
chars = [ch.upper() for ch in "hello"] print(chars) # Output: # ['H', 'E', 'L', 'L', 'O']

Here, each character is converted to uppercase.

Why Use List Comprehension?

  • Concise: Replace multi-line loops with one line.

  • Readable: Express list transformations clearly.

  • Efficient: Often faster than loops due to optimized implementation.

List Comprehension vs Loop

Using a loop

Python 
nums = [1, 2, 3] squares = [] for x in nums: squares.append(x * x) print(squares) # Output: # [1, 4, 9]

Using a list comprehension

Python 
squares = [x * x for x in nums] print(squares) # Output: # [1, 4, 9]

Both produce the same result, but comprehension is more concise.

Important Notes

  • Conditions in comprehensions filter elements.

  • Nested comprehensions create combinations.

  • Comprehensions can include expressions and function calls.

  • Overly complex comprehensions may hurt readability — use wisely.