Generators only run once

What this problem practises

Generators produce values lazily and can be consumed exactly once. That makes them memory-friendly for big sequences, and genuinely confusing the first time one is iterated again and yields nothing.

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

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Keep going

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