Recursive list flattening

What this problem practises

Recursion needs a base case that actually triggers. A missing or drifting base case ends in RecursionError, and the call-stack order decides which value comes back first when predictions are made by eye.

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 recursion problem in this topic.
• Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.