What set operations does Python support? | Interview Question

Q: What set operations does Python support?

**Target Audience:** Beginners preparing for Python interviews — you need to know what a list is; school-level Venn diagrams help. **Difficulty:** Easy **Python version:** Checked on CPython 3.11 and the page runner (Pyodide 0.25.1 = CPython 3.11.3). > **How to read this doc:** Each idea is explained in plain language first, then tagged with `📌 Interview term:` — the precise word an interviewer expects you to use. Every output shown was produced by actually running the code on CPython 3.11 and in the page runner (Pyodide 0.25.1, which is CPython 3.11.3); the **Verified** section quotes that run exactly. ## 1. Why This Even Matters — A Story First Two friends plan a party and each writes a guest list. Who is invited by both? Who did only one of them think of? Is everyone on Ana's list also on Ben's? You could compare the lists name by name, or you could draw two overlapping circles and read the answers off the diagram. A Python set **is** those circles: it holds each item once, it d

The problem

Question presented to candidate: "What operations does a Python set support? Show me how you would find what two collections have in common, what is missing from one of them, and explain when a set is the right data structure in the first place."

What a strong answer should cover:

A set is an unordered collection of unique, hashable items backed by a hash table, so x in s is O(1) on average versus O(n) for a list.
The four operations with their operators and methods: union |, intersection &, difference -, symmetric difference ^; the methods accept any iterable (and several of them), the operators need sets.
Comparisons are containment tests: <= subset, < proper subset, >=/> superset, isdisjoint — sets are only partially ordered, so a < b and b < a can both be False.
In-place forms (|=, &=, -=, ^=, update, intersection_update, ...) and single-item add, remove (raises KeyError) versus discard (silent).
frozenset is the immutable, hashable variant (usable as a dict key or set element); {} is an empty dict; 1, 1.0 and True count as one element; dedup with set() loses order, dict.fromkeys keeps it.

Code / implementation expected: Yes — a short snippet comparing two collections (common, missing, extra items).

set

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for Python interviews — you need to know what a list is; school-level Venn diagrams help. Difficulty: Easy Python version: Checked on CPython 3.11 and the page runner (Pyodide 0.25.1 = CPython 3.11.3).

How to read this doc: Each idea is explained in plain language first, then tagged with 📌 Interview term: — the precise word an interviewer expects you to use. Every output shown was produced by actually running the code on CPython 3.11 and in the page runner (Pyodide 0.25.1, which is CPython 3.11.3); the Verified section quotes that run exactly.

1. Why This Even Matters — A Story First

Two friends plan a party and each writes a guest list. Who is invited by both? Who did only one of them think of? Is everyone on Ana's list also on Ben's? You could compare the lists name by name, or you could draw two overlapping circles an

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8 min read · 10 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

PythonUnion, intersection, difference, comparisons and in-place updates

Practice

PythonDedup, comparing collections, frozenset and the common gotchas

Practice

Discussion

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