Python List Comprehension

List comprehensions are a quick, easy way to create lists in Python. They offer a short, readable alternative to creating and editing lists using the lambda function and/or for-loops, and they’re considered a more Pythonic way to create and combine the functional methods filter(), map().

From the docs:

List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.

Example 1: How it works

It’s easy, honestly. You just have to adjust the way that you express your logic. List comprehensions read a little more like English.

Here’s an example. I’m stealing the first example from the docs.

Say we want to do this:

>>> numbers = range(10)
>>> squares = []
>>> for x in numbers:
...     squares.append(x**2)
>>> squares
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

To create this list using list comprehension, we could instead do this:

>>> numbers = range(10)
>>> squares2 = [x**2 for x in numbers]
>>> squares2
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

In English, this reads a bit like:

  • Set squares2 to a list
  • Fill it with x**2 for every x in the range of 0 to 9

To compare this to a traditional function:

  • In the first part of the list comprehension, you put the operation(s) you want to do, whose return values will populate the list you’re making
  • The second part should iterate over the iterable from which you want to create your new list

Map as list comprehension

This is exactly what we do with the map() function. We take a list numbers, we map an effect across every item in that list – here the effect is raising each element to the power of 2 – and we make a new list of the return values of doing this.

The difference is that, instead of passing map a function that raises each element to the power of 2, and an iterable from which to feed into the function, we tell the list comprehension what to do with elements it receives, and we define a for-loop to do the iterating and value passing.

List comphrension has been deemed a more Pythonic way of mapping an effect onto elements in a list than the built-in map() function.

For my purpose, I’m happy to abide by best practices regardless of the philosophy, but if you care about the debate, I believe that people prefer list comprehensions to functional methods for two reasons:

  1. List comprehensions read more like math/logic expressions
  2. They also read a little more like English

Example 2

I have often seen people coming from other languages map the int() function on a list of strings they are reading in as strings. This happens all of the time reading in inputs for Hacker Rank problems.

>>> string_input = ['10', '20', '3']
>>> int_input = map(int, string_input)
>>> int_input
[10, 20, 3]

List comprehensions are much better choice to do this in my opinion:

>>> string_input = ['10', '20', '3']
>>> int_input = [int(s) for s in string_input]
>>> int_input
[10, 20, 3]

Filter as a list comprehension

I’m going to switch to strings for a change of pace.

Let’s say we have a block of text:

text = "Hello, my name is Reed. I am 28 years old. I am a hack-hack-hackin away at the Recurse Center in NYC. I play around in Python and I like words and vinegar."

I haven’t covered the split() method. It splits a string into a list on a specified token.

text_list = text.split()  # It defaults to splitting the string over spaces
>>> text_list
['Hello,', 'my', 'name', 'is', 'Reed.', 'I', 'am', '28', 'years', 'old.', 'I', 'am', 'a', 'hack-hack-hackin', 'away', 'at', 'the', 'Recurse', 'Center', 'in', 'NYC.', 'I', 'play', 'around', 'in', 'Python', 'and', 'I', 'like', 'words', 'and', 'vinegar.']

Now we have a list of the words from the original text, which I stored in a variable called text_list. Now we can use this in some examples of list comprehensions.

Example 3

Let’s say we want to filter this list and only look at words longer than 4 characters. Continuing from the code above:

>>> long_words = [words for words in text_list if len(words) > 4]  # Notice our boolean expression
>>> long_words
['Hello,', 'Reed.', 'years', 'hack-hack-hackin', 'Recurse', 'Center', 'around', 'Python', 'words', 'vinegar.']

In English:

  • Make long_words a list of words for the words in text_list if the length of the words is greater than 4.

What if we want to look at frequency instead of size, the words that appear at least twice in the text.

>>> freq_words = [words for words in text if text.count(words) >= 2]
>>> freq_words
['I', 'am', 'I', 'am', 'in', 'I', 'in', 'and', 'I', 'and']

I think that exercises are a good way to get a feel for these things.

If you agree:

Exercise 10

Take the list from before:

FILES = ["picture1", "georgi_paws_everywhere", "custom_wreath"]

Map the “.jpg” extension onto each string using a list comprehension.


Exercise 11

This is an exercise from Exercism. My assignment to you, is to complete this using a list comprehension.

Rna Transcription Write a program that, given a DNA strand, returns its RNA complement (per RNA transcription).

Both DNA and RNA strands are a sequence of nucleotides.

The four nucleotides found in DNA are adenine (A), cytosine (C), guanine (G) and thymine (T).

The four nucleotides found in RNA are adenine (A), cytosine (C), guanine (G) and uracil (U).

Given a DNA strand, its transcribed RNA strand is formed by replacing each nucleotide with its complement:

G -> C C -> G T -> A A -> U


Bonus Exercise

David Branner, a fellow Recurser, recently wrote a short article about Python’s list comprehensions.

Go read it.

He includes an example with a clever, Pythonic way to check if a number is negative. Do you understand how it works?

Written on May 4, 2016