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Software Design for Programmers in era of Agentic Coding

Part III - Patterns

Part II gave you the grammar of design. Part III gives you the phrasebook: named patterns, each a crystallized application of those principles (Chapter 8 already hinted that the patterns are the principles, named). Knowing a pattern’s name is leverage — it is the compression that lets you ask your agent for an entire structure in three words, and the shared vocabulary that lets it answer precisely.

The three families

Section titled “The three families”

The patterns sort into the three families the literature has used since the Gang of Four — themselves a piece of vocabulary worth owning, because naming the family is itself compression (“I need a structural pattern to make these two interfaces fit”):

  • Creational — patterns about making objects: who decides which concrete type to build, and how. (Chapter 10: Factories, and the demoted Singleton.)
  • Structural — patterns about composing objects into larger shapes without rigid coupling. (Chapter 11: Adapter & Façade · Chapter 12: Composite & Decorator.)
  • Behavioral — patterns about assigning responsibility and coordinating how objects collaborate. (Chapter 13: Strategy & Template Method · Chapter 14: Observer & State · Chapter 15: Iterator & Visitor.)
FamilyChapterPatterns
Creational10Factory function · Factory Method · Abstract Factory · Singleton (demoted)
Structural11Adapter · Façade
Structural12Composite · Decorator
Behavioral13Strategy · Template Method
Behavioral14Observer · State
Behavioral15Iterator · Visitor

How these chapters are organized

Section titled “How these chapters are organized”

Within each family we pair patterns by the problem they solve, so you meet alternatives side by side — Strategy next to Template Method, Adapter next to Façade — and learn to choose, not just to apply. Every pattern is taught Pythonic-first: the classical class-based form, then the lighter form Python’s functions, protocols, generators, and match often make better, then the decision rule for which to reach for. And every pattern carries its counterweight — the over-application failure mode this book never lets out of its sight.