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Set removes duplicates automatically
🧠 Predict the output Topic: sets Difficulty: easy Free
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: easy — the fix or the trick is normally a single line, and a careful read should be enough.
The challenge
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 — read it, then submit your prediction:
numbers = [1, 2, 2, 3, 3, 3]
unique = set(numbers)
print(len(unique))
print(3 in unique)Understand the rule
Building a set discards duplicates on insertion: set([1, 2, 2, 3]) holds three elements, and len() reflects the deduplicated count. The second rule these puzzles test: sets are unordered, so printed element order is not guaranteed to match insertion order — a correct prediction states the members and the length, not a specific sequence.
How to trace it
- List the input values and strike out every repeat — first occurrence stays, later ones vanish silently.
- Count the survivors for any len() call.
- Watch equality subtleties: 1, 1.0 and True collapse to one element because they compare equal.
- For membership tests (x in s), answer from the deduplicated set.
Common wrong fixes
- Predicting an exact printed order for a set; only membership and size are guaranteed.
- Forgetting that {} is an empty dict — the empty set must be written set().
- Missing cross-type duplicates: {1, True, 1.0} has exactly one element.
Rule to remember
Sets keep one copy of each equal value and promise no order. Predict membership and count — and remember set() (not {}) is the empty set.
💼 Learning path: Interview Practice
Problem 13 of 16 in this path. Work them in order — each one builds on the rule before it.
← Tuple immutabilityNext in path: Default parameter values →
More sets problems
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.