Lesson 17 — List comprehensions

Build lists in one line

A list comprehension creates a new list from an existing sequence in one expression: [expression for x in items]. Add an if at the end to keep only some items. It does the same job as a for loop with append(), just shorter.

words = ["hi", "hello", "yo"]
long_words = [w.upper() for w in words if len(w) > 2]
print(long_words)  # ['HELLO']

Why it matters: comprehensions are everywhere in real Python code, so reading and writing them is a core skill.