List comprehensions

2 min read 7 runnable examples Quiz included

A list comprehension builds a new list from another collection in a single line. It is one of Python's most loved features.

From loop to comprehension

Here is a loop that builds a list of squares:

Python

squares = []
for n in range(1, 6):
    squares.append(n * n)
print(squares)

And the same thing as a comprehension:

Python

squares = [n * n for n in range(1, 6)]
print(squares)

Read it as: "a list of n * n, for each n in range(1, 6)".

The pattern is always [expression for item in collection].

More examples

Python

names = ["asha", "leo", "mei"]
print([name.title() for name in names])
print([len(name) for name in names])

prices = [100, 250, 80]
print([round(p * 1.18, 2) for p in prices])   # add 18% tax

Filtering with if

Add if at the end to keep only some items:

Python

numbers = [4, 11, 8, 3, 15, 6]
evens = [n for n in numbers if n % 2 == 0]
print(evens)

words = ["apple", "kiwi", "banana", "fig"]
long_words = [w for w in words if len(w) > 4]
print(long_words)

Choosing a value with if/else

To transform every item but choose the value, put a conditional expression at the start:

Python

numbers = [4, 11, 8, 3]
labels = ["even" if n % 2 == 0 else "odd" for n in numbers]
print(labels)

Note the difference:

[x for x in items if condition] filters, so the result can be shorter.
[a if condition else b for x in items] transforms, so the result is the same length.

Converting input

Comprehensions are perfect for turning a line of input into numbers:

Python

numbers = [int(x) for x in input().split()]
print(numbers, "sum =", sum(numbers))

Nested comprehensions

Python

grid = [[row * col for col in range(1, 4)] for row in range(1, 4)]
print(grid)

flat = [x for row in grid for x in row]
print(flat)

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