zope.event

zope.event provides a tiny, dependency-free event system for Python. It offers an event-publishing API for applications that want to emit events without knowing who is listening, and a very simple synchronous dispatch mechanism that calls a global list of subscriber functions in order.

Deliberately minimal, zope.event is designed as a foundation on which richer event-dispatching systems are built. The type-based dispatching in zope.component, for example, layers on top of it. Despite its small size it is one of the most-downloaded packages on PyPI, pulled in transitively across the Zope and Python ecosystems.

What You Get

• A global subscribers list of callables that receive published events
• A notify(event) function that dispatches an event to all subscribers in order
• A classhandler helper for class-based subscriber registration
• Zero runtime dependencies and a fully synchronous execution model
• A stable foundation reused by zope.component and other dispatchers

Common Use Cases

• Emitting application events without coupling to specific subscribers
• Serving as the low-level dispatch primitive under zope.component
• Building a custom, more sophisticated event-dispatch system on top
• Adding simple in-process hooks or lifecycle notifications to a library

Under The Hood

Architecture — The package is intentionally minimal: a module-level subscribers list holds callables, and notify(event) iterates that list, calling each subscriber with the event object synchronously and in registration order. There is no type matching or filtering here — that responsibility is pushed up to consumers such as zope.component, which registers a handler in subscribers and implements its own type-based dispatch on top.

Tech Stack — Pure Python with no runtime dependencies, supporting a wide range of CPython (and PyPy) versions. It is packaged with the standard Zope toolchain, tested via GitHub Actions, and documented on Read the Docs.

Code Quality — A mature, long-lived Zope Foundation project (200+ commits, 30 contributors) with continuous integration and Sphinx documentation. The code surface is small enough to audit at a glance, and its ~7.5M weekly downloads reflect deep transitive use across the ecosystem.

API Design — The API is about as small as an event system can be: import notify, append a callable to subscribers, done. This simplicity is the point — it makes zope.event trivial to learn and a clean substrate for building more capable dispatchers without imposing any policy of its own.