Calling a method that doesn't exist

What this problem practises

Python strings are immutable sequences. "Fixing" a string always produces a new one, so the return value has to be captured; slicing rules apply, and every method — upper(), strip(), split(), join() — behaves differently from the in-place mutation you might be picturing.

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: 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

• Python debugging guide — a systematic method for finding bugs like this one fast.
• Browse every strings problem in this topic.
• Common Python errors and what they actually mean, plus the Python glossary for any term used on this page.