def METHOD_NAME(self, key: HathorEvents, fn: PubSubCallable) -> None:
    """Subscribe to a specific event.
    :param key: Name of the key to which to subscribe.
    :type key: string
    :param fn: A function to be called when an event with `key` is published.
    :type fn: function
    """
    if fn not in self._subscribers[key]:
        self._subscribers[key].append(fn)