事件驱动系统

def solve():
    class EventEmitter:
        def __init__(____):
            self._listeners = {}
        def on(____):
            if event not in self._listeners:
                self._listeners[event] = []
            self._listeners[event].append(____)
        def emit(____):
            return '____'.join(fn(____) for fn in self._listeners.get(____))
    em = EventEmitter()
    em.on(____)
    em.on(____)
    print(em.emit(____))