code: def call(self, event, *event_args):
          
          # XXX It would be a lot better for debugging if this error was raised
          # when registering the second listener instead of when the event is
          # emitted.
          assert self.num_listeners(event) == 1, (
              'Expected exactly 1 event listener, found %d listeners' %
              self.num_listeners(event))
          listener = self._listeners[event][0]
          args = list(event_args) + list(listener.user_args)
          return listener.callback(*args)
docs: Call the single registered listener for ``event``.
  
          The listener will be called with any extra arguments passed to
          :meth:`call` first, and then the extra arguments passed to :meth:`on`
  
          Raises :exc:`AssertionError` if there is none or multiple listeners for
          ``event``. Returns the listener's return value on success.