List comprehensions

Build a useful list

Lesson 6 of 20 · 8 min · Module 2: Loops & functions

A list comprehension is a compact way to build a new list from an existing collection. It combines iteration, optional filtering, and an output expression.

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By the end, you’ll be able to…

Read a comprehension from its loop
Add a filter without changing the order
Prefer a readable loop for more complex work

The idea

The pattern is [expression for item in items if condition]. Leave out the condition when you want to transform every item.

scores = [30, 80, 50]
passing = [score for score in scores if score >= 50]
print(passing) # [80, 50]

01Translate to a familiar loop

The comprehension corresponds to creating an empty list, looping through scores, checking each score, and appending the values that pass. The result is a new list.

02Keep the expression small

A comprehension is useful when one expression and one condition describe the work. If you need several actions or complicated error handling, a regular for loop is often clearer.

Your turn

Define passing_scores(scores) to return a new list containing only scores at least 50. Keep their input order and leave the original list unchanged.

What the checks look for

Selects passing scores in order
Handles empty and failing lists
Returns a new list

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Take this with you

A comprehension describes the values you want in the result.