def get_extensions_path(service_plugins=None):paths = collections.OrderedDict()# Add Neutron core extensionspaths[core_extensions.__path__[0]] = 1if service_plugins:# Add Neutron *-aas extensionsfor plugin in service_plugins.values():neutron_mod = provider_configuration.NeutronModule(plugin.__module__.split('.')[0])try:paths[neutron_mod.module().extensions.__path__[0]] = 1except AttributeError:# Occurs normally if module has no extensions sub-modulepass# Add external/other plugins extensionsif cfg.CONF.api_extensions_path:for path in cfg.CONF.api_extensions_path.split(":"):paths[path] = 1LOG.debug("get_extension_paths = %s", paths)# Re-build the extension stringpath = ':'.join(paths)return path

get_extensions_path函数构造了一个paths字典，extensions的路径为key，values均为1。这个paths包括三个部分：

获取到extensions的path后，调用PluginAwareExtensionManager的构造函数：

class ExtensionManager(object):"""Load extensions from the configured extension path.See tests/unit/extensions/foxinsocks.py for anexample extension implementation."""def __init__(self, path):LOG.info(_LI('Initializing extension manager.'))self.path = pathself.extensions = {}self._load_all_extensions()

class ExtensionManager(object):def _load_all_extensions(self):"""Load extensions from the configured path.The extension name is constructed from the module_name. If yourextension module is named widgets.py, the extension class within thatmodule should be 'Widgets'.See tests/unit/extensions/foxinsocks.py for an example extensionimplementation."""for path in self.path.split(':'):                   # 分别处理path中的各个extensions目录if os.path.exists(path):self._load_all_extensions_from_path(path)else:LOG.error(_LE("Extension path '%s' doesn't exist!"), path)def _load_all_extensions_from_path(self, path):# Sorting the extension list makes the order in which they# are loaded predictable across a cluster of load-balanced# Neutron Serversfor f in sorted(os.listdir(path)):                                  # 将path（extension目录）下的所有文件进行排序并遍历。try:LOG.debug('Loading extension file: %s', f)mod_name, file_ext = os.path.splitext(os.path.split(f)[-1])ext_path = os.path.join(path, f)if file_ext.lower() == '.py' and not mod_name.startswith('_'):  # 将所有.py文件（除__init__.py）作为一个extension添加到self.extensions中。mod = imp.load_source(mod_name, ext_path)ext_name = mod_name[0].upper() + mod_name[1:]               # extensions的实现类（ext_name）是对应模块名（mod_name）的首字母大写。new_ext_class = getattr(mod, ext_name, None)                # 根据ext_name到相应模块中获取extensions的实现类，没有获取到则抛出异常。if not new_ext_class:LOG.warning(_LW('Did not find expected name ''"%(ext_name)s" in %(file)s'),{'ext_name': ext_name,'file': ext_path})continuenew_ext = new_ext_class()                                   # 实例化extensions目录下各.py文件中的extensions实现类。self.add_extension(new_ext)                                 # 将各个extensions的实例添加到ExtensionManager的extensions属性中。except Exception as exception:LOG.warning(_LW("Extension file %(f)s wasn't loaded due to ""%(exception)s"),{'f': f, 'exception': exception})

可以看出，各个extension的加载就是在_load_all_extensions函数中进行的。总而言之，这个过程就是到各个extensions目录（上面已经分析过这些目录分为三个部分）中遍历每个.py文件（除了__init__.py），每个.py文件对应一个extension，该extension对应的实现类是.py文件的文件名的首字母大写。获取到各个extension的对应类后实例化，并将实例添加到PluginAwareExtensionManager.extensions变量中。

完成各个extension的加载后，在PluginAwareExtensionManager的构造函数中，还要检查是否所有extensions都被加载：