Python List Comprehension

Basic Syntax

The general form is [expression for item in iterable]. Python evaluates the expression for every item and collects the results into a new list.

squares = [n * n for n in range(1, 6)]
print(squares)  # [1, 4, 9, 16, 25]

Filtering With if

Add an if condition at the end to keep only items that pass a test — this replaces a for loop combined with an if statement and an append() call.

nums = range(1, 11)
evens = [n for n in nums if n % 2 == 0]
print(evens)  # [2, 4, 6, 8, 10]

if / else in the Expression

To choose between two values for every item (rather than filtering), place if/else before the for clause.

labels = ["even" if n % 2 == 0 else "odd" for n in range(5)]
print(labels)

Nested Comprehensions

You can nest comprehensions to flatten a nested list, or use them to build a grid. Keep nesting to one level for readability — beyond that, a regular for loop is usually clearer.

matrix = [[1, 2], [3, 4], [5, 6]]
flat = [num for row in matrix for num in row]
print(flat)  # [1, 2, 3, 4, 5, 6]

Comprehension vs a for Loop

A comprehension is shorthand for a for loop that appends to a list — it is not a different feature, just a more concise, often faster syntax for the same job.

# Using a for loop
squares = []
for n in range(1, 6):
    squares.append(n * n)

# Using a list comprehension
squares = [n * n for n in range(1, 6)]