class Emitter:

    '''Event emitter mixin class.'''

    def add_events(self, *event_names):

        '''Add event names which may be registered by user.

        This should be called only by subclasses.

        This serves also as initializer, other methods of Emitter

        will not work if add_events was not called.

        '''

        if not hasattr(self, '_event_handlers'):

            self._event_handlers = dict()

        for event_name in event_names:

            self._event_handlers[event_name] = []

    def connect(self, event_name, handler):

        '''Connect event handler to event name.

        Add handler to the end of handler list.

        '''

        if event_name in list(self._event_handlers.keys()):

            self._event_handlers[event_name].append(handler)

            raise KeyError('Unknown event: %s', event_name)

    def disconnect(self, event_name, handler=None):

        '''Remove event handler from the list.

        if event_name in list(self._event_handlers.keys()):

                self._event_handlers[event_name].remove(handler)

                self._event_handlers[event_name][:] = []

            raise KeyError('Unknown event: %s', event_name)

    def handle(self, event_name, *args, **kwargs):

        '''Emit the event.

        Call all handlers from event's handler list,

        starting from first added handler.

        Return True when one of the handlers returns True,

        False otherwise.

        for handler in self._event_handlers[event_name]: