List comprehensions

From loops to comprehensions

Transforming one list into another is the single most common move in working Python: pull one field out of every record, convert the units on every reading, normalize every string before saving it. Comprehensions are the tool Python added because that pattern shows up everywhere, and they are the first thing reviewers look for when judging whether Python code is fluent.

In Python for Beginners you built lists the long way: start with an empty list, loop, and append each item.

squares = []
for n in [1, 2, 3, 4]:
    squares.append(n * n)

That pattern is so common that Python has a one-line shortcut for it: the list comprehension. It builds a brand-new list from any sequence you can loop over.

squares = [n * n for n in [1, 2, 3, 4]]

Read it left to right as: build a list of `n n, for each n in this sequence*. Same result, one line, and no empty-list setup or append` call.

The expression can be anything

The part before for is a normal Python expression. It runs once per item, and its result lands in the new list. You can call functions, do string work, whatever you like:

names = ["ada", "linus", "grace"]
caps = [name.upper() for name in names]
# ["ADA", "LINUS", "GRACE"]

Two rules of thumb:

• Use a comprehension when you are transforming every item of a sequence into a new list.
• Keep using a plain for loop when the body has side effects (printing, saving files) or needs several statements.