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Chapter 19: Set, Frozen Set, and GC

Need to check if a million user IDs exist in your database? Need to find common connections between social networks? Need to eliminate duplicates from a massive dataset? Sets are your answer.

Python's set and frozenset types offer O(1) average-time lookup—meaning checking if an item exists is nearly instant, regardless of collection size. While lists require checking every element (O(n) time), sets use hash-based storage to jump directly to the answer. This makes sets essential for performance-critical operations like deduplication, membership testing, and mathematical set operations.

But sets aren't magic—they require understanding hashing (how Python converts objects to lookup keys) and immutability (why only unchangeable objects can be set members). You'll also explore garbage collection (GC)—Python's automatic memory management system that frees unused objects and prevents memory leaks.

By the end of this chapter, you'll build a Memory Profiler tool that tracks object creation and deletion in your programs—integrating sets, frozensets, and garbage collection into a practical debugging aid.

What You'll Learn​

This chapter teaches hash-based data structures and memory management through 6 progressive lessons:

Foundation: Sets and Uniqueness​

  • Lesson 1: Set Basics — Understand what sets are, create them with type hints (set[int]), and grasp the uniqueness property and hashability requirement
  • Lesson 2: Set Operations — Perform mathematical operations (union, intersection, difference) and write set comprehensions for filtered collections
  • Lesson 3: Set Internals & Hashing — Understand how hash functions enable O(1) lookup, why immutability is required, and when sets outperform lists dramatically

Immutable Variants and Memory Management​

  • Lesson 4: Frozensets — Use immutable sets as dictionary keys and nested set members; choose between set and frozenset based on requirements
  • Lesson 5: Garbage Collection — Understand Python's automatic memory management through reference counting, handle circular references, and use the gc module for profiling

Integration Capstone​

  • Lesson 6: Memory Profiler Capstone — Design and build a working tool that tracks object creation/deletion using sets, frozensets, and the gc module