Recursive function missing a base case

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 a fix-the-bug problem: a short program is given in its broken state, and the task is to repair the code so it produces the required output. The interactive editor runs your fixed version in the browser and checks it for you — no setup, no local interpreter.

Difficulty: medium — expect to hold two interacting rules in your head at once, such as a boundary combined with a mutation.

The challenge

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Keep going

• Python debugging guide — a systematic method for finding bugs like this one fast.
• 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.