What is list comprehension and how to use it?

List comprehension is a concise way to create a new list from an existing collection in a single line.

Syntax:

[expression for item in iterable]

Examples:

# Squares of numbers
squares = [x ** 2 for x in range(6)]
# [0, 1, 4, 9, 16, 25]

# Equivalent with a loop
squares = []
for x in range(6):
    squares.append(x ** 2)

With condition (filtering):

# Only even numbers
evens = [x for x in range(10) if x % 2 == 0]
# [0, 2, 4, 6, 8]

With if/else condition (transformation):

labels = ["even" if x % 2 == 0 else "odd" for x in range(5)]
# ['even', 'odd', 'even', 'odd', 'even']

Nested comprehensions:

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

Analogs for other types:

# Dict comprehension
squares_dict = {x: x ** 2 for x in range(5)}

# Set comprehension
unique_lengths = {len(word) for word in ["cat", "dog", "fox"]}

Important:

Avoid overusing complex nested comprehensions — if the expression is hard to read, use a regular loop instead.