Proposal to modify ExtensionPoint to allow specialisation

[trac-form@…]

By turning ExtensionPoint into a descriptor it moves the decision of what to return, into the ExtensionPoint object. This allows one to sub-class ExtensionPoint and customise its behaviour. It has no negative side-effects that I can think of.

To illustrate how this could be useful, the following example lets the user choose a single active interface implementation out of the set of available extensions.

This is useful for user-configurable extension selection, say of a database backend, or a UI front-end; any situation where only a single implementation should be used.

class SingletonExtensionPoint(ExtensionPoint):
    def __get__(self, instance, owner):
        xtnpt = instance._extension_points.get(self.name)
        if xtnpt:
            extensions = ComponentMeta._registry.get(xtnpt.interface, [])
            seek = instance.compmgr._active_components.get(xtnpt.interface, None)
            if seek:
                for extension in extensions:
                    if extension.__name__ == seek:
                        return extension
                raise AttributeError("can't find active interface '%s' named '%s'" % (xtnpt.interface.__name__, seek))
            else:
                raise AttributeError("no component active for '%s' interface" % xtnpt.interface.__name__)

class AnInterface(Interface):
    def do_stuff():
        pass

class ImplementationOfAnInterface(Component):
    implements(AnInterface)

    def do_stuff(self):
        print "ImplementationOfAnInterface.do_stuff()"

class M(ComponentManager, Component):
    _active_components = {}

    aninterface = SingletonExtensionPoint(AnInterface)

    def set_active(self, interface, component):
        self._active_components[interface] = component

m = M()
m.set_active(AnInterface, 'ImplementationOfAnInterface')
i = m.aninterface

[trac-form@…]

Implementation of proposal

[trac-form@…]

Slight bug-fix in the SingletonExtensionPoint example:

return extension

should be

return instance.compmgr[extension]

[Christopher Lenz]

Yeah, changing ExtensionPoint to a descriptor is a good idea.

[trac-form@…]

1.0? Boooooooooooo :(

[Christian Boos]

Also, the patch/proposal doesn't seem to be complete, I tested it on top of the FlexibleWikiPageNames branch, and it raises the following error:

Traceback (most recent call last):
   ...
File "D:\Workspace\install\lib\python\trac\devel\Lib\site-packages\trac\wiki\api.py", line 204, in get_wiki_syntax
    p = self.wikipagenames_providers
  File "D:\Workspace\install\lib\python\trac\devel\Lib\site-packages\trac\core.py", line 63, in __get__
    seek = instance.compmgr._active_components.get(xtnpt.interface,
AttributeError: 'Environment' object has no attribute '_active_components'

The Environment.set_active method seems to be missing as well.

Also, what would you suggest for a standard way to assign the appropriate implementation. There should be a simple (automated?) way to get them from the TracIni, or otherwise pick a default implementation.

[trac-form@…]

In the example above, M is the ComponentManager and implements set_active and _active_components. The patch is intentionally minimal, behaving identically to the existing code, but allows the possibility of alternative ExtensionPoint semantics, such as the example SingletonExtensionPoint above. I can provide a more complete implementation if interested.

SingletonExtensionPoint was just an example, but if something similar were to be implemented it would require modifications to Environment, perhaps with a default method in ComponentManager.

I had envisaged using TracIni as you suggest; it would seem to be quite intuitive.

Perhaps something like the following?

[selected_components]
IDatabaseProvider = SQLiteDB
ISourceControlSystem = SubversionSCS

[Christian Boos]

Have a look at r2563, which integrates the proposed implementation for ExtensionPoint and introduces a SingleExtensionPoint which implements a similar way for selecting a single implementation for a given interface to what I already did as a special case, in the FlexibleWikiPageNames branch.

Everything works fine.

[trac-form@…]

I like it. It definitely makes life easier.

[Christopher Lenz]

[2603] makes extension points descriptors as to your proposal.

I don't think a generic singleton-style extension point implementation makes much sense right now, and selecting one component by disabling the others doesn't seem that elegant IMO.

[Matthew Good]

An implementation of the SingletonExtensionPoint has been added in [2866].