What is the difference between a set and a frozenset?
Set is mutable; frozenset is immutable and hashable
Both set and frozenset are unordered collections of unique hashable elements and support the same mathematical set operations. The key difference is mutability: set can add and remove elements, so it is not hashable and cannot be a dict key or another set's element. frozenset is immutable after construction, so it is hashable if its elements are hashable and can be used as a dict key or set member.
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Use set when you need to mutate membership during the program.
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Use frozenset when you need an immutable, hashable signature, such as a cache key or permission set.
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Trade-off: frozenset gives safety and hashability; set gives mutation. You can convert with set(fs) or frozenset(s).
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Common mistake: thinking set elements can be any object. Elements must be hashable, so lists and dicts cannot be set members.
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Version note: set and frozenset have been built-in since Python 2.4; their core semantics are stable.
Scenario Questions
0-2 years experience
- 1You need to store unique tags and never change them. Which set type?
- 2Can a set contain a list? Why or why not?
2-5 years experience
- 1You need a cache key made of several tags. How do you build it?
- 2You have a frozenset and need to add one tag temporarily. What do you do?
5-8 years experience
- 1You are designing permissions as sets. How do you compare roles and handle inheritance?
- 2You need thread-safe read-only set shared across workers. What do you use?
8+ years experience
- 1Design a permission system where frozensets are used as immutable cache keys and sets are used for mutations, with auditability.
- 2You need to deduplicate millions of frozenset signatures. How do you optimize memory?
Follow-up Questions
- Can a frozenset contain a list? Why or why not?
- How would you compute a cache key from a dynamic set of tags?