registry = {}

def registerPlugin(name, plugin):
    registry[name] = plugin

class IPlugin(zope.interface):
    def doStuff():
        """ does stuff """

class PluginA(object):
    implements(IPlugin)
    def doStuff(self):
        print str(self), "is doing stuff"

class PluginB(PluginA):
    implements(IPlugin)
    print str(self), "is doing stuff"

registerPlugin("A", PluginA())

registerPlugin("B", PluginB())


registry["A"].doStuff()
registry["B"].doStuff()