singleton

id	creational/singleton
name	Singleton
aliases	single-instance
guide_url	https://python-patterns.guide/gang-of-four/singleton/
problem	Guarantee a class has exactly one instance and give the whole program access to it.
symptoms	shared config object one connection pool global registry only one instance
verdict	prefer-alternative
caveats	You almost always want the Global Object pattern instead: build the instance at import time in a module and import it. Singleton classes make tests order-dependent — state leaks between tests through the hidden instance. The GoF __new__ dance still runs __init__ on every call in Python; a factory function avoids the trap entirely.
stdlib_sightings	None Ellipsis NotImplemented

Singleton

One instance for the whole process, reachable from anywhere. Verdict: prefer an alternative — a module already is a singleton; write a module-level instance, or a Shared accessor when construction must wait. Keep a reset seam for tests.

Where	What
pattern/	The importable code: Shared — lazy build, one instance, reset() seam
docs/	Fundamentals · Implementation guide · External examples
examples/app_config/	Mini-project: process-wide settings behind get_settings()
tests/	Behavioral tests for the pattern and the mini-project

uv run python -m patterns.creational.singleton.examples.app_config.main