Python list comprehensions

If, like me, you learned python and programming at the same time, you may have missed out on advanced features that, while awesome in python, won’t carry over to other languages you’ll “graduate” to using. One of these features is a list comprehension. It lets you in a compact (and readable) way write loops that take a list and return a list, and does not require lambda syntax. They let you write this:

list_squared = []
for x in [1, 2, 3, 4, 5]:
    list_squared.append(x ** 2)

like this:

[x ** 2 for x in [1, 2, 3, 4, 5]]

This really shines when you’re transforming and extracting data – you can also stick an ‘if’ at the end, turning this:

odds_squared = []
for x in [1, 2, 3, 4, 5]:
    if x % 2:
        odds_squared.append(x ** 2)

into this

[x ** 2 for x in [1, 2, 3, 4, 5] if x % 2]

It might not be a huge improvement in amount of code (or LOC, since you’ll want to break complex list comprehensions across multiple lines) but it saves you from potential mistakes, typing ‘append’, and (importantly) having to declare and use another variable name. I consider that a win.

If you found this post exciting, generator expressions will blow your mind.