About the Package

This package enables extensible and context-sensitive dispatch to different code implementations that depend both on the annotated type of arguments and on predicates that are fulfilled by arguments.

Specifically, these dispatch decisions are arranged in a manner different than with blocks of if/elif or match/case statements, and also differently from inheritance hierarchies that resolve to a narrowest descendant type containing a given method.

Numerous developers have created a version of a multimethods for Python (see History of Dispatch Concepts). Most or all of those use decorators, or other conventions, to attach multiple implementations to the same global name, and switch between implentations at call time within an ordinary-looking function.

I have decided here on a slightly different API. A "dispatcher" is a namespace in which multiple callable names may live, and calling each one makes a runtime dispatch decision. The general intention in this design is that these namespaces (classes, behind the scenes) can associate related functionality, and the collection of names and implementations in a namespace can all be imported by importing the one namespace object.

A default dispatcher named Dispatcher can be imported directly, but normally a factory function will generate new ones. In the Usage example, a namespace called nums is created (e.g. for numeric functions with multiple implementations), but a real problem might create others called events or datasets or customers.

The advantage of having a namespace object that maintains dispatchable implementations is that that object itself is indefinitely extensible. Within your application code that imports, e.g., the num namespace object, you can add many new function names and/or implementations for the already defined names.

A full application might have multipe namespace objects, perhaps each imported from a different upstream developer. While a namespace can do more than a mere module, in many ways importing a namespace object resembles importing a module (which is, after all, also just a namespace at heart).

Resources

I gave a presentation on a beta version of gnosis-dispatch and the background concepts that motivated creation of this package at PyCon Africa 2025. That talk was titled Multiple and Predicative Dispatch (the LibreOffice and PDF versions are retained in the repository):

Conference Talk