examples.md

Observer — where it lives outside this repo

Cited, real implementations to study (or point an agent at) when designing or reviewing event/notification code.

Python standard library

• concurrent.futures.Future.add_done_callback. Register any callable on a future; it fires on completion — and fires immediately if the future already resolved, a late-subscriber decision worth copying. docs.python.org/3/library/concurrent.futures.html#concurrent.futures.Future.add_done_callback
• asyncio callbacks. The event loop's core currency: loop.call_soon, future/task done-callbacks — completion observers driving the whole async machine. docs.python.org/3/library/asyncio-eventloop.html · docs.python.org/3/library/asyncio-future.html

Major ecosystems

• Django signals. post_save, request_finished, custom signals — the canonical Python pub/sub, with @receiver as decorator subscription. Its docs' warning that signals make flow "harder to follow" is the pattern's main cost, stated by its biggest user. docs.djangoproject.com/en/stable/topics/signals/
• blinker — the standalone signals library Flask builds on; named signals, weak references to subscribers (an answer to the lapsed-listener leak). blinker.readthedocs.io (unverified)
• traitlets — observable attributes (observe/@observe) powering Jupyter's configuration system; the property-setter idiom grown into a framework. traitlets.readthedocs.io (unverified)

Outside Python, for contrast

• DOM addEventListener — the same shape every web developer already knows: subscribe callables to a subject's named events; removeEventListener is the lapsed-listener chore made visible.

What to notice across all of them

Each one had to answer the two questions the classic diagram skips: what order (Django: registration order; DOM: registration order per phase) and what happens when a listener throws (Django propagates unless you use send_robust; the DOM isolates). This module's Signal makes exactly those two decisions explicit parameters.