Sets

Introduction

Sets are unordered collections of unique, hashable items — O(1) membership, union, intersection. Perfect for deduplication.

Understanding the topic

Core concepts to understand:

• {1, 2, 3} or set().
• x in s is O(1) avg.
• Operations: | union, & intersect, - diff.
• frozenset is the immutable variant.

Syntax reference

Visual flow / code:

a = {1, 2, 3}
b = {3, 4, 5}

print(a | b)   # {1,2,3,4,5}
print(a & b)   # {3}
print(a - b)   # {1, 2}
print(a ^ b)   # {1,2,4,5} symmetric diff

# Deduplicate
unique = set([1, 1, 2, 2, 3])  # {1,2,3}

# Membership test (fast)
if user_id in admin_set: ...

Execution workflow

Real-world use

Sets are the go-to for membership tests on large datasets. Replace x in big_list (O(n)) with x in big_set (O(1)).

Best practices

• Use sets for membership tests.
• Set ops for algebra over collections.
• frozenset for set-of-sets.

Common mistakes

• Set elements must be hashable — no lists/dicts inside.

Hands-on exercise

Interview preparation — practice these questions:

• Set vs list lookup time?
• What's a frozenset?
• Why must set items be hashable?

Summary

In summary: O(1) membership, dedup, set algebra. Hashable items only.