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What is the difference between a set and a list in Python?

Python/Codingeasy0.4 min readPremium
Frequency
Low
Asked at 2 companies
Category
179
questions in Python/Coding
Difficulty Split
127E|24M|28H
in this category
Total Bank
1,863
across 7 categories
Asked at these companies
AltimetrikInfosys
Interview Pro Tip

Red Flag: Using a list for membership checks in a loop—O(n²). Pro-Move: 'I use sets for lookups and dedup; for order-preserving dedup I use dict.fromkeys(seq) or OrderedDict.'

Key Concepts Tested
python
Expert AnswerPremium
83 wordsInterview-ready
**List**: Ordered; allows duplicates; indexable; O(n) lookup. **Set**: Unordered; unique elements only; hash-based; O(1) average lookup. **Why it matters**: Set membership and deduplication are fast; lists preserve order and support duplicates. **Scalability trade-off**: Sets use more memory per element (hash + overhead); lists are compact. For large dedup, set wins on lookup; for ordered data, list is required. **Cost implication**: Repeated `x in list` is O(n) per check; `x in set` is O(1)....
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