extension.py

class Category:
    def __init__(self, name, interface):
        self.name = name
        self.interface = interface
        self.classes = {}
        self.instances = {}
    
    def register(self, cls):
        if self.interface is not None:
            if not is_implementation(cls, self.interface):
                raise ValueError,\
                        "the specified cls doesn't agree to the interface: %s" % \
                         (str(self.interface))
        class_name = _get_class_name(cls)
        self.classes[class_name] = cls

    def get_extensions(self):
        '''return a list of ready-to-use extension instances'''
        return [self._instance_of(class_name) for class_name in self.classes.keys()]

    def _instance_of(self, class_name):
        '''Given a class name, will return a ready-to-use instance. 
        Every "trick" (hey, only if necessary), will be done here.
        '''
        if class_name in self.instances:
            return self.instances[class_name]

        instance = self.classes[class_name]()
        self.instances[class_name] = instance
        return instance

    def get_default(self):
        '''return ONE extension instance. It will be chosen with preferences'''
        #put here your choosing logic (preferences)
        available = self.get_extensions()
        if available:
            return available[0] #that's just a test: we should choose better
        return None


_categories = {} #'CategoryName': Category('ClassName')

def category_register(category, interface=None):
    '''Add a category'''
    _categories[category] = Category(category, interface)

def register(category_name, cls):
    '''Register cls as an Extension for category. 
    If the class doesn't agree to the required interface, raises ValueError.
    If the category doesn't exist, it creates it(but returns False).
    It doesn't instanciate that class immediately.
    @return False if the category didn't exist. Probably you made a mistake, True otherwise.
    '''
    get_category(category_name).register(cls)

def get_category(category_name):
    '''Get a Category object'''
    return _categories[category_name]

def get_extensions(category_name):
    '''return a list of ready-to-use extension instances'''
    return get_category(category_name).get_extensions()

def get_default(category_name):
    return get_category(category_name).get_default()

def is_implementation(cls, interface_cls):
    '''Check if cls implements all the methods provided by interface_cls.
    Note: every cls implements None.
    '''
    for method in [attribute for attribute in dir(interface_cls)
            if not attribute.startswith('_')]:
        if not hasattr(cls, method):
            return False
    return True

def _get_class_name(cls):
    '''Returns the full name of a class: module.class
    For instances, call get_full_name(self.__class__)'''
    return '.'.join([cls.__module__, cls.__name__])