Set union and intersection

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 an output-prediction problem: read the snippet carefully and write down exactly what it prints before anything runs. These questions are interview staples because they test whether you know the language rules, or merely recognise their syntax.

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

• Output-prediction strategies — how to predict tricky snippets without running them.
• 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.