Document apptools.type_registry

[corranwebster]

This is a comparatively recent addition to apptools. It should be documented with examples of the problems it is idesigned to solve.

[rkern]

There are a number of places where you want to specify some kind of object for a given type (gracefully handling subtypes, lazy registration, etc.). For example, in IPython, I want to register a pretty-printer function for numpy arrays. Or in Traits UI, I might want to make an InstanceEditor implementation that looks up a Controller/ModelView from one of these registries to wrap around a model object. I use (a vendorized version of) this code in qt_binder to map Qt widget types to Binder classes.

It is similar to functools.singledispatch and our adaptation subsystem except that the mapped object is entirely general, not just a function that will be called on the object or an adapter wrapped around it. It's a light-weight implementation of the concept, and the registry object can be explicitly managed (i.e. no hidden global state unless if you decide to hide it somewhere).