text: from txaws.server.exception import APIError
  
  
  class Registry(object):
      """Register API L{Method}s. for handling specific actions and versions"""
  
      def __init__(self):
          self._by_action = {}
  
      def add(self, method_class, action, version=None):
          """Add a method class to the regitry.
  
          @param method_class: The method class to add
          @param action: The action that the method class can handle
          @param version: The version that the method class can handle
          """
          by_version = self._by_action.setdefault(action, {})
          if version in by_version:
              raise RuntimeError("A method was already registered for action"
                                 " %s in version %s" % (action, version))
          by_version[version] = method_class
  
      def check(self, action, version=None):
          """Check if the given action is supported in the given version.
  
          @raises APIError: If there's no method class registered for handling
              the given action or version.
          """
          if action not in self._by_action:
              raise APIError(400, "InvalidAction", "The action %s is not valid "
                             "for this web service." % action)
          by_version = self._by_action[action]
          if None not in by_version:
              # There's no catch-all method, let's try the version-specific one
              if version not in by_version:
                  raise APIError(400, "InvalidVersion", "Invalid API version.")
  
      def get(self, action, version=None):
          """Get the method class handing the given action and version."""
          by_version = self._by_action[action]
          if version in by_version:
              return by_version[version]
          else:
              return by_version[None]
  
      def scan(self, module, onerror=None, ignore=None):
          """Scan the given module object for L{Method}s and register them."""
          from venusian import Scanner
          scanner = Scanner(registry=self)
          kwargs = {"onerror": onerror, "categories": ["method"]}
          if ignore is not None:
              # Only pass it if specified, for backward compatibility
              kwargs["ignore"] = ignore
          scanner.scan(module, **kwargs)
  
      def get_actions(self):
          """
          Get a list of all action names.
          """
          return self._by_action.keys()
  
      def get_versions(self, action_name):
          """
          Get a list of versions supported by the given action name.
          """
          return self._by_action[action_name].keys()
  