回调函数

def apply_async(func, args, , callback):
# Compute the result
result = func(args)

# Invoke the callback with the result
callback(result)

调用

def print_result(result):
... print('Got:', result)
...
def add(x, y):
... return x + y
...
apply_async(add, (2, 3), callback=print_result)
Got: 5
apply_async(add, ('hello', 'world'), callback=print_result)
Got: helloworld

协程处理回调

还有另外一个更高级的方法，可以使用协程来完成同样的事情：

def make_handler():
sequence = 0
while True:
result = yield
sequence += 1
print('[{}] Got: {}'.format(sequence, result))
对于协程，你需要使用它的 send() 方法作为回调函数，如下所示：

handler = make_handler()
next(handler) # Advance to the yield
apply_async(add, (2, 3), callback=handler.send)
[1] Got: 5
apply_async(add, ('hello', 'world'), callback=handler.send)
[2] Got: helloworld