What is list comprehension?

List comprehensions are an easy way to create lists (based on existing lists).

Its much easier to write a one liner than it is to write a for loop, just for creating a list. This _ one liner_ is called a list comprehension.

Related Course: Complete Python Programming Course & Exercises

Introduction

In Python, the syntax of list comprehension is:

list_variable = [x for x in iterable]

Assume we want to create a list containing 100 numbers. Manually that would be a lot of typing work. So we would use a for loop, right?

We can define a for loop to fill the list.

numbers = []

for i in range(0,100):
    numbers.append(i)

print(numbers)

That works, but it’s to much work. Instead you can use list comprehension.

You can replace all the code above with a one liner, which is how we obtain the same result:

numbers = [ x for x in range(100) ]

yes, inside the list we use a loop and the range() function

This is also useful if you want to create large lists.

List Comprehensions

Assume we want all the square roots to 100:

import math

numbers = [ math.sqrt(x) for x in range(100) ]
print(numbers)

You can also use it like this:

>>> h = [ letter for letter in 'hello' ]
>>> h
['h', 'e', 'l', 'l', 'o']
>>> 

An if statement can be inside a list comprehension

>>> number_list = [ x for x in range(20) if x % 2 == 0]
>>> print(number_list)
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18]

It can be used on a list of strings too:

>>> player_list = [ 'alice','xena','bob','veronica' ]
>>> sub_list = [player for player in player_list if player != 'bob']
>>> sub_list
['alice', 'xena', 'veronica']
>>> 

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