Class method missing cls

What this problem practises

Object-oriented bugs hide in state: class attributes shared by every instance, mutable defaults declared in __init__ signatures, and methods that mutate self when the caller expected a copy.

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