Python List Comprehension

A list comprehension builds a new list from an existing one in a single, readable line. It replaces the common “empty list, loop, append” pattern and is one of the features that makes Python code short.

From loop to comprehension

Here is the familiar pattern: start with an empty list, loop, and append.

numbers = [1, 2, 3, 4, 5]squares = []for n in numbers:    squares.append(n * n)print(squares)

[1, 4, 9, 16, 25]

The comprehension says the same thing in one line. Read it as “n times n, for each n in numbers“:

numbers = [1, 2, 3, 4, 5]squares = [n * n for n in numbers]print(squares)

[1, 4, 9, 16, 25]

The general form is [expression for item in iterable].

Filter with if

Add if condition at the end to keep only some items.

scores = [45, 82, 67, 91, 38, 74]passed = [s for s in scores if s >= 50]print(passed) words = ["api", "", "ui", "  ", "db"]clean = [w for w in words if w.strip()]print(clean)

[82, 67, 91, 74]
['api', 'ui', 'db']

Choose a value with if-else

To transform every item one of two ways, put a conditional expression at the front. Note the different position from the filter above.

scores = [45, 82, 67, 91, 38]labels = ["pass" if s >= 50 else "fail" for s in scores]print(labels)

['fail', 'pass', 'pass', 'pass', 'fail']

Work with strings

sentence = "Python makes testing simple"print([w.upper() for w in sentence.split()])print([len(w) for w in sentence.split()])print([ch for ch in "a1b2c3" if ch.isdigit()])

['PYTHON', 'MAKES', 'TESTING', 'SIMPLE']
[6, 5, 7, 6]
['1', '2', '3']

Nested comprehensions

Two for clauses work like nested loops, read from left to right:

pairs = [(x, y) for x in [1, 2, 3] for y in "ab"]print(pairs) matrix = [[1, 2, 3], [4, 5, 6]]flat = [n for row in matrix for n in row]print(flat) transposed = [[row[i] for row in matrix] for i in range(3)]print(transposed)

[(1, 'a'), (1, 'b'), (2, 'a'), (2, 'b'), (3, 'a'), (3, 'b')]
[1, 2, 3, 4, 5, 6]
[[1, 4], [2, 5], [3, 6]]

Set and dict comprehensions

Use { } to build a set, and {key: value ...} to build a dict.

emails = ["a@x.com", "b@y.com", "c@x.com"]domains = {e.split("@")[1] for e in emails}print(sorted(domains)) prices = {"pen": 10, "book": 120, "bag": 650}with_tax = {item: round(p * 1.18, 2) for item, p in prices.items()}print(with_tax) expensive = {k: v for k, v in prices.items() if v > 100}print(expensive)

['x.com', 'y.com']
{'pen': 11.8, 'book': 141.6, 'bag': 767.0}
{'book': 120, 'bag': 650}

Generator expressions

With round brackets you get a generator expression. It produces values one at a time instead of building a list, which saves memory when you only need to consume the values once, for example in sum():

print(sum(n * n for n in range(1, 101)))print(any(s < 40 for s in [45, 82, 38]))

338350
True

The Generators lesson explains how this works.

Check your understanding

data = [3, -1, 0, 8, -5]print([abs(x) for x in data if x != 0], [x if x > 0 else 0 for x in data])

[3, 1, 8, 5] [3, 0, 0, 8, 0]

range 1 to 10; words = ["login", "cart", "checkout"]

[4, 16, 36, 64, 100]
{'login': 5, 'cart': 4, 'checkout': 8}

even_squares = [n * n for n in range(1, 11) if n % 2 == 0]print(even_squares) words = ["login", "cart", "checkout"]lengths = {w: len(w) for w in words}print(lengths)

[4, 16, 36, 64, 100]
{'login': 5, 'cart': 4, 'checkout': 8}

Program Practice

Many list programs can be solved in one line with a comprehension. Solve each one with a loop first, then rewrite it.

Python list comprehension FAQ