List comprehensions

Building a list in one line

This shape appears constantly:

squares = []
for n in numbers:
    squares.append(n ** 2)

A list comprehension says the same thing in one line:

squares = [n ** 2 for n in numbers]

Read it left to right: what to collect, then where it comes from. You can add a filter on the end:

evens = [n for n in numbers if n % 2 == 0]

And combine the two: [n ** 2 for n in numbers if n > 0].

Comprehensions are idiomatic Python, but they are not free — once one needs two filters and a conditional expression, an ordinary loop is kinder to whoever reads it next.

Your turn: from words, build long_words containing the words longer than four characters, uppercased. Use a comprehension.

Comprehensions in functions

A comprehension often replaces an entire function body.

Your turn: write clean(values) returning the numbers from values that are above zero, each rounded to the nearest whole number. Keep the original order.