Python List Comprehensions: Syntax, Examples, and Advanced Uses

List comprehensions provide a concise way to create lists in Python by combining iteration, optional filtering, and expression evaluation in a single line.

Basic syntax : [expression for item in iterable if condition] , where expression is applied to each item , for defines the loop, and an optional if filters items.

Basic usage – generate squares of numbers 0‑9:

# Create a list of squares from 0 to 9
squares = [x**2 for x in range(10)]
print(squares)  # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Filtering even numbers:

# Only square even numbers
even_squares = [x**2 for x in range(10) if x % 2 == 0]
print(even_squares)  # [0, 4, 16, 36, 64]

String handling – convert each word to uppercase:

words = ["hello", "world", "python"]
upper_words = [word.upper() for word in words]
print(upper_words)  # ['HELLO', 'WORLD', 'PYTHON']

Nested loops – generate Cartesian product of colors and sizes:

colors = ['red', 'green']
sizes = ['S', 'M', 'L']
combinations = [(color, size) for color in colors for size in sizes]
print(combinations)  # [('red', 'S'), ('red', 'M'), ('red', 'L'), ('green', 'S'), ('green', 'M'), ('green', 'L')]

Combining with zip() – element‑wise addition of two lists:

numbers1 = [1, 2, 3]
numbers2 = [10, 20, 30]
summed = [x + y for x, y in zip(numbers1, numbers2)]
print(summed)  # [11, 22, 33]

Complex expression – compute Fibonacci numbers (first 10):

fibonacci = [0, 1] + [fibonacci[i-1] + fibonacci[i-2] for i in range(2, 10)]
print(fibonacci)  # [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]

Dictionary comprehension – build a dict from two lists:

keys = ['name', 'age', 'city']
values = ['Alice', 25, 'New York']
person_info = {k: v for k, v in zip(keys, values)}
print(person_info)  # {'name': 'Alice', 'age': 25, 'city': 'New York'}

Set comprehension – remove duplicates from a list:

numbers = [1, 2, 2, 3, 4, 4, 5]
unique_numbers = {n for n in numbers}
print(unique_numbers)  # {1, 2, 3, 4, 5}

Multiple conditions – filter numbers ≥10 and divisible by 3:

filtered_numbers = [x for x in range(20) if x >= 10 and x % 3 == 0]
print(filtered_numbers)  # [12, 15, 18]

Using enumerate() – get index and value:

fruits = ['apple', 'banana', 'cherry']
indexed_fruits = [(i, fruit) for i, fruit in enumerate(fruits)]
print(indexed_fruits)  # [(0, 'apple'), (1, 'banana'), (2, 'cherry')]

Applying a function with list comprehension (instead of map() ):

words = ["hello", "world", "python"]
upper_words = [str.upper(word) for word in words]
print(upper_words)  # ['HELLO', 'WORLD', 'PYTHON']

Flatten a 2‑D list:

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

Filter specific characters from a string:

text = "Hello, World! 123"
letters_only = [char for char in text if char.isalpha()]
print(''.join(letters_only))  # HelloWorld

Cartesian product of two lists:

list1 = ['a', 'b']
list2 = [1, 2]
cartesian_product = [(x, y) for x in list1 for y in list2]
print(cartesian_product)  # [('a', 1), ('a', 2), ('b', 1), ('b', 2)]

Conditional mapping using a dictionary:

data = [1, 2, 3, 4, 5]
category_map = {1: 'low', 2: 'low', 3: 'medium', 4: 'high', 5: 'high'}
categories = [category_map[x] for x in data]
print(categories)  # ['low', 'low', 'medium', 'high', 'high']

Count character occurrences in a string:

from collections import Counter
text = "abracadabra"
char_counts = {char: count for char, count in Counter(text).items()}
print(char_counts)  # {'a': 5, 'b': 2, 'r': 2, 'c': 1, 'd': 1}

Generate a list of random integers:

import random
random_numbers = [random.randint(1, 100) for _ in range(10)]
print(random_numbers)

List comprehension with if‑else expression:

numbers = [1, 2, 3, 4, 5]
processed = ['even' if n % 2 == 0 else 'odd' for n in numbers]
print(processed)  # ['odd', 'even', 'odd', 'even', 'odd']

Process file lines – strip whitespace:

with open('example.txt', 'r') as file:
    lines = [line.strip() for line in file.readlines()]
print(lines)

Build a simple 3×3 matrix using nested comprehensions:

matrix = [[i * j for j in range(1, 4)] for i in range(1, 4)]
print(matrix)  # [[1, 2, 3], [2, 4, 6], [3, 6, 9]]