TypeError: unhashable list added to 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: hard — the naive reading is wrong, and the answer depends on tracking state precisely through the snippet.

The challenge

This program crashes with "TypeError: unhashable type: 'list'". Fix the s.add(...) line so it prints [5, 7, 12].

The code runs in PyDebug’s in-browser compiler. Open the problem, run the snippet, watch the exact error or output, and keep iterating until it is right. Signed-in solves count toward XP and your daily streak.

The snippet

The interactive editor pre-fills exactly this code, broken and ready to repair:

s = {7, 5}
s.add([12])
print(sorted(s))

More problems

• sorted() with a key function medium
• Sorting a list with mixed types medium
• TypeError: sorting mixed text + numbers easy

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.