List Comprehensions

Syntax

[expression for item in iterable if condition]

Examples

numbers = [1, 2, 3, 4, 5]

squares = [n ** 2 for n in numbers]
print(squares)  # [1, 4, 9, 16, 25]

# Equivalent traditional loop
squares2 = []
for n in numbers:
    squares2.append(n ** 2)

numbers = range(1, 21)

evens = [n for n in numbers if n % 2 == 0]
print(evens)  # [2, 4, 6, ..., 20]

words = ["hi", "hello", "hey", "greetings"]
long_words = [w for w in words if len(w) > 3]
print(long_words)  # ['hello', 'greetings']

numbers = [1, 2, 3, 4, 5, 6]

labels = ["even" if n % 2 == 0 else "odd" for n in numbers]
print(labels)  # ['odd', 'even', 'odd', 'even', 'odd', 'even']

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

flattened = [num for row in matrix for num in row]
print(flattened)  # [1, 2, 3, 4, 5, 6, 7, 8, 9]

grid = [[r * c for c in range(3)] for r in range(3)]
print(grid)  # [[0, 0, 0], [0, 1, 2], [0, 2, 4]]

Best practices

• Use list comprehensions for simple transformations and filters - switch back to a regular loop once logic gets complex or hard to read on one line
• Avoid nesting more than two levels deep in a single comprehension - it quickly becomes unreadable
• Use a generator expression (parentheses instead of brackets) when you only need to iterate once and want to save memory
• Remember a comprehension always builds the entire list in memory immediately, unlike a generator

At a glance

Purpose

Scripting and general-purpose applications

File extension

.py

Runs in

Python interpreter

Usually used with

Python standard library and packages

Specifications & further reading

• Python language reference

Related Python documentation