def apply_async(func, args, *, callback):
    result = func(*args)
    callback(result)

def print_result(result):
    print(result)

def add(x, y):
    return x + y

# 如果只是简单的调用回调函数，那直接用就可以了
apply_async(add, (1, 2), callback=print_result)
apply_async(add, ('hello', 'world'), callback=print_result)

# 但是，如果我想要知道这个函数被调用了几次，携带这些保存的信息返回，可以这样改写：
def make_handler():
    sequence = 0
    def handler(result):
        nonlocal sequence
        sequence += 1
        print('[{}] Got: {}'.format(sequence, result))
    return handler
handler = make_handler()
apply_async(add, (1, 2), callback=handler)
apply_async(add, (1, 2), callback=handler)


# 当然我们也能用协程去搞定它：
# 协程的复习在10.1中，单步调试你就看懂了
def make_handler_v2():
    sequence = 0
    while True:
        result = yield
        sequence += 1
        print('[{}] Got: {}'.format(sequence, result))
handler = make_handler_v2()
next(handler)
apply_async(add, (1, 2), callback=handler.send)
apply_async(add, (1, 2), callback=handler.send)
apply_async(add, (1, 2), callback=handler.send)