code: def on(self, event, listener, *user_args):
          
          self._listeners[event].append(
              _Listener(callback=listener, user_args=user_args))
docs: Register a ``listener`` to be called on ``event``.
  
          The listener will be called with any extra arguments passed to
          :meth:`emit` first, and then the extra arguments passed to :meth:`on`
          last.
  
          If the listener function returns :class:`False`, it is removed and will
          not be called the next time the ``event`` is emitted.