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