composition_over_inheritance

id	principle/composition_over_inheritance
name	Composition Over Inheritance
aliases	subclass-explosion favor-composition
guide_url	https://python-patterns.guide/gang-of-four/composition-over-inheritance/
problem	Vary independent behaviors without one subclass per combination of them.
symptoms	subclass explosion FilteredSocketLogger-style names M x N class combinations mixin soup
verdict	pythonic
caveats	Multiple inheritance, mixins, and dynamically built classes are the guide's 'dodges' — they postpone the explosion instead of ending it. Each independent axis of variation should become its own small object, injected where needed.
stdlib_sightings	logging.Logger logging.Handler logging.Filter

Composition Over Inheritance

One small piece per axis of variation, composed at a single point: M + N pieces instead of M × N subclasses. Verdict: pythonic — the most load-bearing idea behind the rest of this catalog.

Where	What
pattern/	The importable code: Pipeline (the composition point), Filter/Transform/Sink axis aliases, and Logger built on it
docs/	Fundamentals · Implementation guide · External examples
examples/notification_router/	Mini-project: alerts through filter × format × deliver pieces, zero combination subclasses
tests/	Behavioral tests for the pattern and the mini-project

uv run python -m patterns.principle.composition_over_inheritance.examples.notification_router.main