How to use the stevedore.named function in stevedore

def __init__(self, namespace, driver_list, **kwargs):
        super(DriverManager, self).__init__()
        manager = stevedore.named.NamedExtensionManager(
            namespace, driver_list, invoke_on_load=True, **kwargs)

        drivers = {}
        for ext in manager:
            type_ = ext.obj.get_type()
            if type_ in drivers:
                msg = _("driver '%(new_driver)s' ignored because "
                        "driver '%(old_driver)s' is already "
                        "registered for driver '%(type)s'") % {
                            'new_driver': ext.name,
                            'old_driver': drivers[type].name,
                            'type': type_}
                LOG.error(msg)
                raise SystemExit(msg)
            drivers[type_] = ext
        self._drivers = dict((type_, 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(".")]

default=DEFAULT_PLUGINS,
                    help=u._('List of crypto plugins to load.')
                    )
]
CONF.register_group(crypto_opt_group)
CONF.register_opts(crypto_opts, group=crypto_opt_group)
config.parse_args(CONF)

config.set_module_config("crypto", CONF)


def list_opts():
    yield crypto_opt_group, crypto_opts


class _CryptoPluginManager(named.NamedExtensionManager):
    def __init__(self, conf=CONF, invoke_args=(), invoke_kwargs={}):
        """Crypto Plugin Manager

        Each time this class is initialized it will load a new instance
        of each enabled crypto plugin. This is undesirable, so rather than
        initializing a new instance of this class use the PLUGIN_MANAGER
        at the module level.
        """
        crypto_conf = config.get_module_config('crypto')
        plugin_names = self._get_internal_plugin_names(crypto_conf)

        super(_CryptoPluginManager, self).__init__(
            crypto_conf.crypto.namespace,
            plugin_names,
            invoke_on_load=False,  # Defer creating plugins to utility below.
            invoke_args=invoke_args,

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_enforcer(namespace):
    """Find a policy.Enforcer via an entry point with the given namespace.

    :param namespace: a namespace under oslo.policy.enforcer where the desired
                      enforcer object can be found.
    :returns: a policy.Enforcer object
    """
    mgr = stevedore.named.NamedExtensionManager(
        'oslo.policy.enforcer',
        names=[namespace],
        on_load_failure_callback=on_load_failure_callback,
        invoke_on_load=True)
    if namespace not in mgr:
        raise KeyError('Namespace "%s" not found.' % namespace)
    enforcer = mgr[namespace].obj

    return enforcer

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 load_extension_options(self, module):
        # Note that options loaded this way aren't added to _opts_by_module
        loader = stevedore.named.NamedExtensionManager(
            'oslo.config.opts',
            names=(module,),
            invoke_on_load=False
        )
        for ext in loader:
            for group, opts in ext.plugin():
                for opt in opts:
                    if group is None:
                        self._add_opt(opt.dest, 'DEFAULT', opt)
                    else:
                        self._add_opt(group + '/' + opt.dest, group, opt)

        self._opt_names.sort(OptionsCache._cmpopts)

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(".")]

app.register_error_handler(code, _json_error)

    # TODO(kiall): Ideally, we want to make use of the Plugin class here.
    #              This works for the moment though.
    def _register_blueprint(ext):
        app.register_blueprint(ext.plugin)

    # Add all in-built APIs
    mgr = extension.ExtensionManager('designate.api.v1')
    mgr.map(_register_blueprint)

    # Add any (enabled) optional extensions
    extensions = cfg.CONF['service:api'].enabled_extensions_v1

    if len(extensions) > 0:
        extmgr = named.NamedExtensionManager('designate.api.v1.extensions',
                                             names=extensions)
        extmgr.map(_register_blueprint)

    return app