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

Rule Categories

#	Category	Impact	Patterns	When to reach for this group
1	Creational	HIGH	5	Object construction is non-trivial, varies by configuration, or risks tight coupling to concrete classes
2	Structural	HIGH	7	Composing classes/objects into larger structures while keeping parts substitutable
3	Behavioral	HIGH	10	Distributing responsibility and defining how objects collaborate at runtime

How to Use

1. Recognize the shape. Read the Quick Reference below and identify which pattern's intent matches your problem. Most pattern-shaped problems sound like one of the listed phrases.
2. Read the pattern reference. Open references/{category}-{pattern}.md. Confirm intent, then read Applicability and When NOT to Use before adopting.
3. Lead with the idiom. Each "Correct" example shows the Pythonic form first. Adopt it unless you need the class-based structure shown in the Alternative block.
4. Adapt to your domain. The examples use small realistic domains (transports, route planners, document trees). Rename to your terms before merging.
5. Check the relations. Each entry ends with Related Patterns — siblings worth considering for the same problem.

Quick Reference

1. Creational Patterns (object instantiation)

• creational-factory-method — Resolve a concrete class through a registry/dispatch dict. "I want to pick a class by config/string key without an if/elif ladder." — HIGH
• creational-abstract-factory — Produce families of related objects that must match. "Switching one flag must swap a whole coordinated set (button + checkbox)." — MEDIUM-HIGH
• creational-builder — Construct complex objects step by step — in Python a keyword-only dataclass first. "My constructor has 10+ params, or I need staged assembly." — HIGH
• creational-prototype — Clone via copy.deepcopy / dataclasses.replace. "I need another one just like this, with one value changed." — MEDIUM
• creational-singleton — One shared instance via a module global or functools.cache. "I need exactly one config/registry/pool, kept testable." — MEDIUM

2. Structural Patterns (composition)

• structural-adapter — Wrap a class so its interface matches what callers expect. "This library's method names don't match mine and I can't edit it." — HIGH
• structural-bridge — Split abstraction from implementation via composition + Protocol. "Two orthogonal axes and the subclass count is exploding." — MEDIUM
• structural-composite — Treat leaves and trees uniformly via a shared Protocol + recursion. "I have a tree and want one interface for items and groups." — HIGH
• structural-decorator — Stack wrappers (or use @decorator) to add behavior at runtime. "I want to layer logging + caching + compression in any order." — HIGH
• structural-facade — Expose one function/module over a complex subsystem. "I just want convert(file, fmt), not the codec/bitrate dance." — HIGH
• structural-flyweight — Share immutable state via a cached factory + __slots__. "Millions of objects, only a few distinct payloads — out of RAM." — LOW-MEDIUM
• structural-proxy — Stand in via __getattr__ / cached_property to control access. "I need lazy loading / auth / caching without touching the real object." — MEDIUM-HIGH

3. Behavioral Patterns (collaboration)

• behavioral-chain-of-responsibility — Run a request through an ordered list of handlers. "A pipeline of auth/validate/authorize checks I want to reorder." — MEDIUM-HIGH
• behavioral-command — Reify a request as a callable/closure with optional undo. "I need undo/redo, queueing, or one action shared across UI surfaces." — HIGH
• behavioral-iterator — Traverse via __iter__/generators without exposing internals. "I want for x in my_structure to just work." — HIGH
• behavioral-mediator — Route component interaction through one hub. "My widgets all reference each other and nothing is reusable." — MEDIUM
• behavioral-memento — Snapshot/restore state via a frozen dataclass. "I need undo/rollback without exposing private fields." — LOW-MEDIUM
• behavioral-observer — Notify subscriber callbacks on change (often a property setter). "Many objects must react when one value changes — events, reactive UI." — CRITICAL
• behavioral-state — Delegate to polymorphic state objects (or an enum + dispatch). "My class is a state machine with if status == in every method." — MEDIUM-HIGH
• behavioral-strategy — Pass an algorithm as a Callable and swap it at runtime. "Multiple algorithms (sort/route/pay) picked without conditionals." — HIGH
• behavioral-template-method — Fix a skeleton in an ABC; subclasses override steps. "Several classes share an algorithm with a couple of varying steps." — MEDIUM
• behavioral-visitor — Add operations via functools.singledispatch / match. "I need 5 operations across an AST without editing the node classes." — LOW-MEDIUM

How to Choose Between Similar Patterns

Several patterns share a shape but solve different problems. Read each pattern's Related Patterns section, then apply these distinctions:

• Adapter vs. Facade vs. Proxy vs. Decorator — all four wrap a target. Adapter changes the interface. Facade simplifies a subsystem. Proxy keeps the interface and controls access/lifecycle. Decorator keeps the interface and adds behavior recursively.
• Strategy vs. State — both delegate to a swapped object. Strategy variants are independent functions the caller picks. State objects know each other and trigger transitions on the context.
• Strategy vs. Template Method — both vary parts of an algorithm. Strategy uses composition — a Callable swapped at runtime. Template Method uses inheritance — an ABC skeleton fixed at definition time.
• Factory Method vs. Abstract Factory vs. Builder — Factory Method resolves one product (a registry/@classmethod). Abstract Factory returns a family of matching products. Builder assembles one complex product (a keyword-only dataclass, or a fluent builder for staged construction).
• Composite vs. Decorator — both wrap children recursively. Composite aggregates child results. Decorator adds one responsibility and passes through.
• Chain of Responsibility vs. Command vs. Mediator vs. Observer — all connect senders and receivers. CoR passes a request along a list of handlers (and may stop early). Command makes the request a first-class callable. Mediator centralizes many-to-many communication. Observer establishes one-publisher-to-many-subscribers notification.
• Visitor: singledispatch vs. match vs. methods — use functools.singledispatch to add operations over a closed type set without editing the classes; use match when you'd rather keep all cases in one exhaustive function; use plain methods when there's one operation and the type set is small.

References

1. Refactoring Guru — Design Patterns Catalog
2. Refactoring Guru — Python Examples
3. Refactoring Guru — Creational Patterns
4. Refactoring Guru — Structural Patterns
5. Refactoring Guru — Behavioral Patterns