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implementation-design-patterns-python

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Implementation guide for the 22 Gang of Four design patterns in idiomatic modern Python (3.10+), distilled from refactoring.guru. Use when writing, refactoring, or reviewing Python with a pattern-shaped problem — class-explosion from inheritance, conditionals switching on type, tight coupling to concrete classes, tree-shaped models, runtime algorithm selection, undo/redo, state-dependent behavior, or hiding subsystem complexity. Each entry leads with the Pythonic form (functions, dataclasses, Protocol, singledispatch, match, generators, copy/replace) and falls back to the class-based GoF structure only when identity, state, or dispatch require it. Includes intent, applicability (when to use AND when NOT to), a runnable example, steps, pros/cons, and relations. Trigger even when no pattern is named — cues like "too many constructor params," "giant if/elif," "swap behavior at runtime," "need undo," or "walk a tree" are pattern-shaped. Covers all 5 Creational, 7 Structural, and 10 Behavioral GoF patterns.

skills/.experimental/implementation-design-patterns-python/SKILL.md

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# Python Design Patterns Best Practices (Refactoring Guru)

Implementation reference for the 22 Gang of Four design patterns in **idiomatic modern Python (3.10+)**, distilled from refactoring.guru. Each of the **22 pattern files across 3 categories** captures intent, problem, solution, applicability (when to use AND when NOT to), a runnable Python example with output, implementation steps, pros/cons, and relations to sibling patterns.

This is the **Pythonic-first** companion to the TypeScript design-patterns skill. Most GoF patterns shrink to a language feature in Python — a function, a generator, a dataclass, `functools.singledispatch`, a `match` statement. Every entry leads with that idiomatic form and keeps the class-based GoF structure only where identity, stored state, runtime registration, or polymorphic dispatch genuinely earn it.

The patterns are a *vocabulary for structural decisions*, not a prescription. Reach for one only when its applicability criteria match — every entry includes a **When NOT to Use** section to guard against over-engineering, which is the more common failure with this catalog in Python.

## When to Apply

- A constructor has grown to 10+ parameters (telescoping-constructor smell) or subclasses exist only to bake in parameter combinations
- A method branches on `kind`/`type`/`mode`/`status` to pick an algorithm or behavior — a `match` or `if/elif` ladder that grows with each variant
- Integrating an incompatible third-party API, library, or legacy class whose method names don't match your code
- Modeling a tree-shaped domain (file systems, ASTs, UI trees, org charts) where leaves and branches must be treated uniformly