class Event:
  def __init__(self):
    self._subscribers = []

  def sub(self, callback):
    """Subscribe a callback to the event."""
    if callback not in self._subscribers:
      self._subscribers.append(callback)

  def unsub(self, callback):
    """Unsubscribe a callback from the event."""
    if callback in self._subscribers:
      self._subscribers.remove(callback)

  def invoke(self, *args, **kwargs):
    """Invoke all subscribers with the given arguments."""
    for callback in self._subscribers:
      callback(*args, **kwargs)