Week 17 - Pythonic Design Patterns¶

17.1 Conceptual Core¶

The GoF book describes patches around C++ and Java limitations: no first-class functions, no closures, no duck typing, mandatory class hierarchies. Many "patterns" in Python collapse to language features. Some still apply. Knowing which is which is senior-level taste.

17.2 The Catalog, Translated¶

GoF Pattern	Pythonic Form
Strategy	A function passed as an argument. Or a `Protocol`.
Template Method	A function with hooks; `ABC` + `abstractmethod` only when you need enforcement.
Factory / Abstract Factory	A function. Or `__init_subclass__` registry. Or `functools.singledispatch`.
Singleton	A module. `import` is the singleton. `@lru_cache(maxsize=None)` on a constructor for parameterized singletons.
Observer	`signal`/`blinker`, asyncio `Queue`, or just a list of callbacks.
Iterator	Built into the language (`__iter__`).
Decorator (GoF)	Decorators (Python).
Adapter	A function. Or `Protocol` + a thin wrapper class.
Visitor	`functools.singledispatch` for type dispatch; `match` statement for ADTs.
Command	A callable. `functools.partial` for binding.
Chain of Responsibility	Middleware. ASGI/WSGI middleware is exactly this.
State	Functions returning functions; or a `match` over an `Enum`.
Builder	Keyword args + `dataclass`. Rarely a builder class.
Flyweight	`sys.intern` for strings; `__slots__` + class-level constants.
Proxy	`__getattr__`-based forwarding; or `weakref.proxy`.
Composite	A type that contains itself: `tree: Node \| list[Node]`.
Memento	`dataclasses.replace` + immutability.
Mediator	An event bus or a domain service.

The patterns that survive idiomatically: Strategy (via Protocol), Observer (via async queues), Chain (via middleware), Visitor (via singledispatch / match).

17.3 Architectural Patterns¶

Hexagonal / Ports and Adapters. Domain at the core, adapters at the edge (HTTP, DB, message bus). Test the domain in isolation. Architecture Patterns with Python (Percival/Gregory) is the canonical Python treatment.
Repository pattern: abstract persistence behind a Protocol. Tests use an in-memory fake; production uses SQL.
Unit of Work: collect domain mutations, commit atomically. Pairs with SQLAlchemy session.
CQRS-lite: separate read models from write models when their shapes diverge. Don't over-apply.

17.4 Lab - "Refactor a Junk Drawer"¶

Take a 1k-LOC script of mixed responsibilities. Extract: domain/, adapters/, service/, entrypoints/. Write Protocols for the seams.
Add a fake repository for tests; the real one talks to SQLite. Run the same test suite against both.
Document, in a docs/architecture.md, why each module exists and what it depends on.

17.5 Idiomatic & Linter Drill¶

import-linter to enforce the dependency direction (entrypoints → service → domain ← adapters).