Adding an unhashable type to a set

What this problem practises

Sets are unordered containers that drop duplicates on insertion. Membership tests are fast, insertion order is not preserved, and {} builds an empty dict — not an empty set.

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