README.md

id	behavioral/observer
name	Observer
aliases	publish-subscribe listener event-handler
guide_url
problem	Notify interested parties when something changes, without the subject knowing who they are.
symptoms	react to changes event listeners pub/sub on_change callbacks model updates views
verdict	pythonic
caveats	Observers are callables — an Observer ABC with one update() method is a function with extra steps. Decide the failure policy: one raising observer can silence the rest. Notify inside try/except or document that observers must not raise.
stdlib_sightings	concurrent.futures.Future.add_done_callback asyncio.Future

Observer

Broadcast a change to whoever subscribed, in order, without the subject knowing its audience. Verdict: pythonic — observers are callables in a list; the only real design decisions are order and failure policy.

Where	What
pattern/	The importable code: Signal, Subscriber, ErrorPolicy
docs/	Fundamentals · Implementation guide · External examples
examples/order_events/	Mini-project: order pipeline with independent subscribers built on pattern/
tests/	Behavioral tests for the pattern and the mini-project

uv run python -m patterns.behavioral.observer.examples.order_events.main