[PyQt] question about dip plugins

[Darren Dale]

The dip documentation at
http://www.riverbankcomputing.com/static/Docs/dip/plugins_tutorial.html
mentions:

"... When a plugin requests a service the plugin manager will choose
which service is actually used. The plugin does not care about the
particular service, its only concern is that it has an object that
implements the interface. ..."

I am starting work on a project that will simulate spectra based on
physical reference data. There are several databases to choose from,
so in a sense the task is similar to the recipe chooser example in the
plugins tutorial. I would like to be able to compare the results of
one "recipe" with the next. Is it possible for the client to request a
particular service or plugin, rather than simply accept whatever the
plugin manager decides to provide?

Also, from the same tutorial:

@implements(IPlugin)
class RecipeChooserPlugin(Plugin):

Why is it necessary to decorate subclasses of Plugin with
@implements(IPlugin)? Are there cases when subclasses of Plugin would
not implement the IPlugin interface?

Thanks,
Darren