在回调函数中携带额外的状态

---

def apply_async(func, args, *, callback):
    # 执行函数
    result = func(*args)
    # 回调函数介入
    callback(result)
# 例子
def add(x, y):
    return x + y
# (a) 一个简单的回调函数例子
print('# --- Simple Example')
def print_result(result):
    print("Got:", result)
apply_async(add, (2, 3), callback=print_result)
apply_async(add, ('hello', 'world'), callback=print_result)
# (b) 使用绑定方法 （bound method）
print('# --- Using a bound-method')
class ResultHandler:
    def __init__(self):
        self.sequence = 0
    def handler(self, result):
        self.sequence += 1
        print('[{}] Got: {}'.format(self.sequence, result))
r = ResultHandler()
apply_async(add, (2, 3), callback=r.handler)
apply_async(add, ('hello', 'world'), callback=r.handler)
# (c) 使用闭包
print('# --- Using a closure')
def make_handler():
    sequence = 0
    def handler(result):
            # python3里面的nonlocal
        nonlocal sequence
        sequence += 1
        print('[{}] Got: {}'.format(sequence, result))
    return handler
handler = make_handler()
apply_async(add, (2, 3), callback=handler)
apply_async(add, ('hello', 'world'), callback=handler)
# (d) 使用协程
print('# --- Using a coroutine')
def make_handler():
    sequence = 0
    while True:
        result = yield
        sequence += 1
        print('[{}] Got: {}'.format(sequence, result))
handler = make_handler()
next(handler)    # 初始化
apply_async(add, (2, 3), callback=handler.send)
apply_async(add, ('hello', 'world'), callback=handler.send)
# (e) 使用partial
print('# --- Using partial')
class SequenceNo:
    def __init__(self):
        self.sequence = 0
def handler(result, seq):
    seq.sequence += 1
    print('[{}] Got: {}'.format(seq.sequence, result))
seq = SequenceNo()
from functools import partial
apply_async(add, (2, 3), callback=partial(handler, seq=seq))
apply_async(add, ('hello', 'world'), callback=partial(handler, seq=seq))