Squares of even numbers

What this problem practises

Lists are mutable and assigned by reference. That one fact explains most list bugs: aliasing between what look like "copies", append() and sort() returning None, and slice boundaries that disagree with the index arithmetic.

This is an output-prediction problem: read the snippet carefully and write down exactly what it prints before anything runs. These questions are interview staples because they test whether you know the language rules, or merely recognise their syntax.

Difficulty: hard — the naive reading is wrong, and the answer depends on tracking state precisely through the snippet.

The challenge

What will this program print? Type the exact output.

The code runs in PyDebug’s in-browser compiler. Open the problem, run the snippet, watch the exact error or output, and keep iterating until it is right. Signed-in solves count toward XP and your daily streak.

The snippet

The interactive editor pre-fills exactly this code — read it, then submit your prediction:

nums = [5, 9, 12, 9, 10, 7]
squares = [x * x for x in nums if x % 2 == 0]
print(squares)

More lists problems

• Squares of odd numbers hard
• IndexError: pop() from an empty list easy
• List slicing and indexing easy

Keep going

• Output-prediction strategies — how to predict tricky snippets without running them.
• Browse every lists problem in this topic.
• Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.