How to use the stevedore.enabled function in stevedore

self.rebuild_schema_manager = \
            stevedore.enabled.EnabledExtensionManager(
                namespace=self.EXTENSION_REBUILD_NAMESPACE,
                check_func=_check_load_extension('get_server_rebuild_schema'),
                invoke_on_load=True,
                invoke_kwds={"extension_info": self.extension_info},
                propagate_map_exceptions=True)
        if list(self.rebuild_schema_manager):
            self.rebuild_schema_manager.map(self._rebuild_extension_schema,
                                            self.schema_server_rebuild)
        else:
            LOG.debug("Did not find any server rebuild schemas")

        # Look for API schema of server resize extension
        self.resize_schema_manager = \
            stevedore.enabled.EnabledExtensionManager(
                namespace=self.EXTENSION_RESIZE_NAMESPACE,
                check_func=_check_load_extension('get_server_resize_schema'),
                invoke_on_load=True,
                invoke_kwds={"extension_info": self.extension_info},
                propagate_map_exceptions=True)
        if list(self.resize_schema_manager):
            self.resize_schema_manager.map(self._resize_extension_schema,
                                           self.schema_server_resize)
        else:
            LOG.debug("Did not find any server resize schemas")

def __init__(self, init_only=None):
        def _check_load_extension(ext):
            return self._register_extension(ext)

        self.api_extension_manager = stevedore.enabled.EnabledExtensionManager(
            namespace=self.api_extension_namespace(),
            check_func=_check_load_extension,
            invoke_on_load=True,
            invoke_kwds={"extension_info": self.loaded_extension_info})

        mapper = ProjectMapper()

        self.resources = {}

        if list(self.api_extension_manager):
            self._register_resources_check_inherits(mapper)
            self.api_extension_manager.map(self._register_controllers)

        LOG.info("Loaded extensions: %s",
                 sorted(self.loaded_extension_info.get_extensions().keys()))
        super(APIRouterV1, self).__init__(mapper)

def _get_plugins(self):
        """Return dict of resource-plugin class pairs."""
        config_plugins = CONF.manager.plugins
        plugins = {}

        extension_manager = enabled.EnabledExtensionManager(
            check_func=lambda ext: ext.name in config_plugins,
            namespace='blazar.resource.plugins',
            invoke_on_load=False
        )

        invalid_plugins = (set(config_plugins) -
                           set([ext.name for ext
                                in extension_manager.extensions]))
        if invalid_plugins:
            raise common_ex.BlazarException('Invalid plugin names are '
                                            'specified: %s' % invalid_plugins)

        for ext in extension_manager.extensions:
            try:
                plugin_obj = ext.plugin()
            except Exception as e:

def get_extensions(cls, enabled_extensions=None):
        """Load a series of extensions"""

        LOG.debug('Looking for extensions in %s' % cls.__plugin_ns__)

        def _check_func(ext):
            if enabled_extensions is None:
                # All extensions are enabled by default, if no specific list
                # is specified
                return True

            return ext.plugin.get_plugin_name() in enabled_extensions

        mgr = enabled.EnabledExtensionManager(
            cls.__plugin_ns__, check_func=_check_func,
            propagate_map_exceptions=True)

        return [e.plugin for e in mgr]

'%s extension %r disabled through configuration setting',
            namespace, ext.name,
        )
        return False
    if not ext.obj.is_enabled():
        LOG.debug(
            '%s extension %r reported that it is disabled',
            namespace,
            ext.name,
        )
        return False
    LOG.debug('using %s extension %r', namespace, ext.name)
    return True


class ActivatedExtensionManager(enabled.EnabledExtensionManager):
    """Loads extensions based on a configurable set that should be
    disabled and asking each one if it should be active or not.
    """

    def __init__(self, namespace, disabled_names, invoke_on_load=True,
                 invoke_args=(), invoke_kwds={}):

        def local_check_func(ext):
            return should_use_extension(namespace, ext, disabled_names)

        super(ActivatedExtensionManager, self).__init__(
            namespace=namespace,
            check_func=local_check_func,
            invoke_on_load=invoke_on_load,
            invoke_args=invoke_args,
            invoke_kwds=invoke_kwds,

def _load_cluster_plugins(self):
        config_plugins = CONF.plugins
        extension_manager = enabled.EnabledExtensionManager(
            check_func=lambda ext: ext.name in config_plugins,
            namespace='sahara.cluster.plugins',
            invoke_on_load=True
        )

        for ext in extension_manager.extensions:
            if ext.name in self.plugins:
                raise ex.ConfigurationError(
                    _("Plugin with name '%s' already exists.") % ext.name)
            ext.obj.name = ext.name
            self.plugins[ext.name] = ext.obj
            LOG.info("Plugin {plugin_name} loaded {entry_point}".format(
                     plugin_name=ext.name,
                     entry_point=ext.entry_point_target))

        if len(self.plugins) < len(config_plugins):

#    Licensed under the Apache License, Version 2.0 (the "License"); you may
#    not use this file except in compliance with the License. You may obtain
#    a copy of the License at
#
#         http://www.apache.org/licenses/LICENSE-2.0
#
#    Unless required by applicable law or agreed to in writing, software
#    distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
#    WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
#    License for the specific language governing permissions and limitations
#    under the License.
#
from stevedore import enabled


class EnabledExtensionManager(enabled.EnabledExtensionManager):
    """CloudKitty Rating processor manager

    Override default EnabledExtensionManager to check for an internal
    object property in the extension.
    """

    def __init__(self, namespace, invoke_args=(), invoke_kwds={}):

        def check_enabled(ext):
            """Check if extension is enabled.

            """
            return ext.obj.enabled

        super(EnabledExtensionManager, self).__init__(
            namespace=namespace,

def __init__(self, migration_config, engine=None):
        if engine is None:
            if migration_config.get('db_url'):
                engine = sqlalchemy.create_engine(
                    migration_config['db_url'],
                    poolclass=sqlalchemy.pool.NullPool,
                )
            else:
                raise ValueError('Either database url or engine'
                                 ' must be provided.')

        self._manager = enabled.EnabledExtensionManager(
            MIGRATION_NAMESPACE,
            check_plugin_enabled,
            invoke_args=(engine, migration_config),
            invoke_on_load=True
        )
        if not self._plugins:
            raise ValueError('There must be at least one plugin active.')

def _load_drivers(self):
        manager = stevedore.enabled.EnabledExtensionManager(
            namespace=self.MODULE_DRIVER_NAMESPACE,
            check_func=self._check_extension,
            invoke_on_load=True,
            invoke_kwds={})
        try:
            manager.map(self.add_driver_extension)
        except stevedore.exception.NoMatches:
            LOG.info("No module drivers loaded")

def initialise():
    global _mgr
    if _mgr:
        return

    def client_is_available(client_plugin):
        if not hasattr(client_plugin.plugin, 'is_available'):
            # if the client does not have a is_available() class method, then
            # we assume it wants to be always available
            return True
        # let the client plugin decide if it wants to register or not
        return client_plugin.plugin.is_available()

    _mgr = enabled.EnabledExtensionManager(
        namespace='heat.clients',
        check_func=client_is_available,
        invoke_on_load=False,
        on_load_failure_callback=pluginutils.log_fail_msg)