implementation-design-patterns-python

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
• Adding cross-cutting behavior at runtime — logging, caching, access control, compression — without subclassing
• Selecting an algorithm or behavior variant at runtime from config, user input, or environment
• Implementing undo/redo, history snapshots, transactional rollback, or queueing/scheduling of operations
• Coordinating many objects whose direct mutual references have become tangled — a hub that brokers communication
• Notifying many subscribers when something changes — event systems, reactive data flows
• Reviewing code that smells like a pattern is implicit (a giant if isinstance(...), parallel class hierarchies, copy-pasted algorithm skeletons) — make it explicit, or collapse it to a Python idiom