Mar 30, 2021
I initially thought Python list comprehensions were obtuse and unnecessary, a bit of syntactic sugar I didn’t need, but after spending more time with them my opinion has completely flipped. Now I not only use them as much as I can, but I also want to get the word out for those who might not know about them.
Let’s start with a simple example. Let’s say I want to alter all the items in a list using a function and generate a new list. Our list will be x and our function will be sqrt from the math module.
First up is the straightforward method using a for loop.
>>> from math import sqrt >>> x = range(1000) >>> y =  >>> for i range x: ... y.append(sqrt(i)) >>> y[:5] [0.0, 1.0, 1.4142135623730951, 1.7320508075688772, 2.0, 2.23606797749979]
Now here’s the same example using list comprehension.
>>> y = [sqrt(i) for i in x] >>> y[:5] [0.0, 1.0, 1.4142135623730951, 1.7320508075688772, 2.0, 2.23606797749979]
List comprehension is simply a set of brackets containing an expression followed by a for clause.
Additionally, one or more if clauses can be added to a list comprehension.
>>> z = [sqrt(i) for i in x if i > 50] >>> z[:5] [7.14142842854285, 7.211102550927978, 7.280109889280518, 7.3484692283495345, 7.416198487095663]
List comprehension also allows for the simple creation of lists on the fly, let’s say 1000 random floats.
>>> import random >>> floats = [random.random() for _ in range(1000)] >>> floats[:5] [0.7780346266380861, 0.6197484726418738, 0.8439569186352499, 0.21261969119693835, 0.1844600543795476]
I will continue to use list comprehensions, one more tool to integrate in my Python toolbox. I hope you agree list comprehensions are great, but part of the reason I wanted to talk about them was in order to build up to generators and generator comprehensions. Stay tuned.