List comprehensions with Python (and Examples)

What is list comprehension?

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A list comprehension builds a new list in one line. It is a for loop with the result appended, written on one line. Everything you can do with a for loop you can do here, it is just shorter.

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.

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']
>>> 

The form

[what_to_put_in for item in collection if condition]

The filter at the end is optional. Without it you take every item, with it you keep only the ones that pass.

It works with any iterable: a list, a tuple, a string, the result of range() or a file object.

A comprehension inside a comprehension is possible but gets hard to read fast. If it does not fit on one line, a normal for loop is clearer.

There is a generator version with round brackets instead of square ones, (x for x in data). It gives the values one by one and never builds the full list, which saves memory on big data.

On Python 3 you can also use sum(...), min(...) and max(...) with a generator instead of writing the loop yourself.

The fastest way to learn this is to write it yourself. The PyChallenge exercises run in your browser, no install needed.