List Comprehensions

14 min 30 XP

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Step 1

Learn the idea

Comprehensions build lists in a single expressive line — Python's signature feature:

[x * 2 for x in nums] — transform every item
[x for x in nums if x > 0] — filter items
[x * 2 for x in nums if x > 0] — filter and transform

Read it as: "give me x * 2, for each x in nums, if x > 0". They replace 4-line loop-and-append patterns with one clear line.

Dict comprehensions work too: {w: len(w) for w in words}. Keep comprehensions simple — if it needs two ifs and nesting, use a normal loop.

Where you'll use this

Comprehensions are the mark of fluent Python — code reviewers at top companies flag 4-line append loops that should be one comprehension. They also map directly onto SQL's SELECT-WHERE mental model.

Common mistakes

Nesting too much: a comprehension with multiple fors and ifs is harder to read than the loop it replaced. Two clauses max.
Using one for side effects: [print(x) for x in items] builds a useless list of Nones — use a plain loop when you don't need the result.
Filter goes after the for: [x if x > 0 for x in nums] is a SyntaxError; [x for x in nums if x > 0] is right. (x if cond else y before the for is a different, valid pattern.)

Pro tip

Generator expressions — the same syntax with () — process items lazily without building a list: sum(n * n for n in range(10**6)) uses almost no memory.

Step 2

Watch how it runs — line by line

Press play to watch Python execute this code, one line at a time.

Step 3

Try it yourself

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PYexample.py

⌘ + Enter to run

Output appears here…

Step 4

Pass the challenge +30 XP

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Using one list comprehension, build a list of the cubes of the odd numbers from 1 to 9, and print it. Expected: [1, 27, 125, 343, 729]

Target output

[1, 27, 125, 343, 729]

PYchallenge.py

cubes = [n ** 3 for n in range(1, 10) if n % 2 == 1]
print(cubes)

Run your code to check it…

Step 5

Check your understanding

1. What does [c.upper() for c in 'abc'] produce? 'ABC' ['A', 'B', 'C'] ['abc'] ['ABC']

2. Where does the if filter go in a comprehension? Before the for After the for clause Inside brackets first Comprehensions can't filter

Last step

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Learn the idea

Where you'll use this

Common mistakes

Pro tip

Watch how it runs — line by line

Try it yourself

Pass the challenge +30 XP

Check your understanding

Your notes (saved on this device)

Write Python that does something useful.

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