How to use the stevedore.named.NamedExtensionManager function in stevedore

def __init__(self):
        enabled_ext = cfg.CONF['service:api'].enabled_extensions_admin
        if len(enabled_ext) > 0:
            self._mgr = named.NamedExtensionManager(
                namespace='designate.api.admin.extensions',
                names=enabled_ext,
                invoke_on_load=True)
            for ext in self._mgr:
                controller = self
                path = ext.obj.get_path()
                LOG.info("Registering an API extension at path %s", path)
                for p in path.split('.')[:-1]:
                    if p != '':
                        controller = getattr(controller, p)
                setattr(controller, path.split('.')[-1], ext.obj)

def __init__(self):
        enabled_ext = cfg.CONF['service:api'].enabled_extensions_v2
        if len(enabled_ext) > 0:
            self._mgr = named.NamedExtensionManager(
                namespace='designate.api.v2.extensions',
                names=enabled_ext,
                invoke_on_load=True)
            for ext in self._mgr:
                controller = self
                path = ext.obj.get_path()
                for p in path.split('.')[:-1]:
                    if p != '':
                        controller = getattr(controller, p)
                setattr(controller, path.split('.')[-1], ext.obj)

if parsed_args.modules:
        for module_name in parsed_args.modules:
            module = _import_module(module_name)
            if module:
                for group, opts in _list_opts(module):
                    opts_by_group.setdefault(group, []).append((module_name,
                                                                opts))

    # Look for entry points defined in libraries (or applications) for
    # option discovery, and include their return values in the output.
    #
    # Each entry point should be a function returning an iterable
    # of pairs with the group name (or None for the default group)
    # and the list of Opt instances for that group.
    if parsed_args.libraries:
        loader = stevedore.named.NamedExtensionManager(
            'oslo.config.opts',
            names=list(set(parsed_args.libraries)),
            invoke_on_load=False,
            on_load_failure_callback=raise_extension_exception
        )
        for ext in loader:
            for group, opts in ext.plugin():
                opt_list = opts_by_group.setdefault(group or 'DEFAULT', [])
                opt_list.append((ext.name, opts))

    for pkg_name in pkg_names:
        mods = mods_by_pkg.get(pkg_name)
        mods.sort()
        for mod_str in mods:
            if mod_str.endswith('.__init__'):
                mod_str = mod_str[:mod_str.rfind(".")]

self.publisher_id = publisher_id
        if retry is not None:
            self.retry = retry
        else:
            self.retry = conf.oslo_messaging_notifications.retry

        self._driver_names = ([driver] if driver is not None else
                              conf.oslo_messaging_notifications.driver)

        if topics is not None:
            self._topics = topics
        else:
            self._topics = conf.oslo_messaging_notifications.topics
        self._serializer = serializer or msg_serializer.NoOpSerializer()

        self._driver_mgr = named.NamedExtensionManager(
            'oslo.messaging.notify.drivers',
            names=self._driver_names,
            invoke_on_load=True,
            invoke_args=[conf],
            invoke_kwds={
                'topics': self._topics,
                'transport': self.transport,
            }

def api_app(conf):
    cfg.CONF(args=[], project='monasca')
    log_levels = (cfg.CONF.default_log_levels)
    cfg.set_defaults(log.log_opts, default_log_levels=log_levels)
    log.setup('monasca')

    dispatcher_manager = named.NamedExtensionManager(
        namespace=DISPATCHER_NAMESPACE,
        names=cfg.CONF.dispatcher,
        invoke_on_load=True,
        invoke_args=[cfg.CONF])

    if not list(dispatcher_manager):
        LOG.error('Failed to load any dispatchers for %s' %
                  DISPATCHER_NAMESPACE)
        return None

    # Create the application
    app = resource_api.ResourceAPI()

    # add each dispatcher to the application to serve requests offered by
    # each dispatcher
    for driver in dispatcher_manager:

def _get_opt_default_updaters(namespaces):
    mgr = stevedore.named.NamedExtensionManager(
        'oslo.config.opts.defaults',
        names=namespaces,
        warn_on_missing_entrypoint=False,
        on_load_failure_callback=on_load_failure_callback,
        invoke_on_load=False)
    return [ep.plugin for ep in mgr]

if parsed_args.modules:
        for module_name in parsed_args.modules:
            module = _import_module(module_name)
            if module:
                for group, opts in _list_opts(module):
                    opts_by_group.setdefault(group, []).append((module_name,
                                                                opts))

    # Look for entry points defined in libraries (or applications) for
    # option discovery, and include their return values in the output.
    #
    # Each entry point should be a function returning an iterable
    # of pairs with the group name (or None for the default group)
    # and the list of Opt instances for that group.
    if parsed_args.libraries:
        loader = stevedore.named.NamedExtensionManager(
            'oslo.config.opts',
            names=list(set(parsed_args.libraries)),
            invoke_on_load=False,
            on_load_failure_callback=raise_extension_exception
        )
        for ext in loader:
            for group, opts in ext.plugin():
                opt_list = opts_by_group.setdefault(group or 'DEFAULT', [])
                opt_list.append((ext.name, opts))

    for pkg_name in pkg_names:
        mods = mods_by_pkg.get(pkg_name)
        mods.sort()
        for mod_str in mods:
            if mod_str.endswith('.__init__'):
                mod_str = mod_str[:mod_str.rfind(".")]

if parsed_args.modules:
        for module_name in parsed_args.modules:
            module = _import_module(module_name)
            if module:
                for group, opts in _list_opts(module):
                    opts_by_group.setdefault(group, []).append((module_name,
                                                                opts))

    # Look for entry points defined in libraries (or applications) for
    # option discovery, and include their return values in the output.
    #
    # Each entry point should be a function returning an iterable
    # of pairs with the group name (or None for the default group)
    # and the list of Opt instances for that group.
    if parsed_args.libraries:
        loader = stevedore.named.NamedExtensionManager(
            'oslo.config.opts',
            names=list(set(parsed_args.libraries)),
            invoke_on_load=False,
            on_load_failure_callback=raise_extension_exception
        )
        for ext in loader:
            for group, opts in ext.plugin():
                opt_list = opts_by_group.setdefault(group or 'DEFAULT', [])
                opt_list.append((ext.name, opts))

    for pkg_name in pkg_names:
        mods = mods_by_pkg.get(pkg_name)
        mods.sort()
        for mod_str in mods:
            if mod_str.endswith('.__init__'):
                mod_str = mod_str[:mod_str.rfind(".")]

def _get_import_flows(**kwargs):
    # NOTE(flaper87): Until we have a better infrastructure to enable
    # and disable tasks plugins, hard-code the tasks we know exist,
    # instead of loading everything from the namespace. This guarantees
    # both, the load order of these plugins and the fact that no random
    # plugins will be added/loaded until we feel comfortable with this.
    # Future patches will keep using NamedExtensionManager but they'll
    # rely on a config option to control this process.
    extensions = named.NamedExtensionManager('glance.flows.import',
                                             names=['ovf_process',
                                                    'convert',
                                                    'introspect'],
                                             name_order=True,
                                             invoke_on_load=True,
                                             invoke_kwds=kwargs)

    for ext in extensions.extensions:
        yield ext.obj

def _get_driver_opts_loaders(namespaces, driver_option_name):
    mgr = stevedore.named.NamedExtensionManager(
        namespace='oslo.config.opts.' + driver_option_name,
        names=namespaces,
        on_load_failure_callback=on_load_failure_callback,
        invoke_on_load=False)
    return [(e.name, e.plugin) for e in mgr]