python-antipatterns-cheatsheet

Anti-pattern cheatsheet

The GoF book was written for C++ and Java in 1994. Some of its patterns address language constraints those languages had (no first-class functions, no modules-as-namespaces, mandatory new) that Python doesn't. Agents still import those shapes into Python, usually as ceremony.

This is the quick reference for the imported patterns that show up in real Python reviews. It is not a GoF encyclopedia.

The cheatsheet

Singleton — anti-pattern

Symptom: class Logger: _instance = None; def __new__(cls): if cls._instance is None: ...; or Logger.instance() as the canonical accessor; or @singleton decorator.

Why it fights Python: Python has modules. A module is loaded once. A module-level variable IS a singleton. There's no new keyword to prevent — there's just construction, which you can choose not to do.

For services with app-lifetime state (DB pool, HTTP client), use dependency injection — create once in lifespan, store on app.state, inject through Depends. Singletons hide their dependencies; DI makes them explicit and testable.

Use instead:

• Module-level constants for true constants
• Dependency injection for services and pools (python-fastapi-sa-app-setup)

# Don't
class StripeClient:
    _instance: "StripeClient | None" = None
    @classmethod
    def instance(cls) -> "StripeClient":
        if cls._instance is None:
            cls._instance = cls(...)
        return cls._instance

# Do — init in lifespan, inject via Depends
async def init_stripe_client(app: FastAPI, config: StripeConfig) -> None:
    app.state.stripe_client = StripeClient(...)

def get_stripe_client(request: Request) -> StripeClient:
    return request.app.state.stripe_client

See: python-fastapi-sa-app-setup, python-external-client

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Abstract Factory — anti-pattern

Symptom: class AbstractEmailClientFactory: @abstractmethod def create(...); concrete factory subclasses for each "family" of products.

Why it fights Python: Classes are first-class objects — you can pass the class itself, or store classes in a dict, and the dispatch becomes a one-line lookup. No AbstractFactory → ConcreteFactory → Product hierarchy needed.