應(yīng)用場(chǎng)景：

• 文件的讀寫
• 數(shù)據(jù)庫(kù)的讀寫操作
• Flask的上下文管理

上下文管理協(xié)議：當(dāng)使用with語句時(shí)擒抛，解釋器會(huì)自動(dòng)調(diào)用 __enter__,__exit__

class Sample:
    def __enter__(self):
        print('enter')  #進(jìn)入資源
        return self
    def __exit__(self, exc_type, exc_val, exc_tb):
        print('exit')  #釋放資源
    def do_something(self):
        print('do something')

with Sample() as sample:   
    sample.do_something()

輸出

enter
do something
exit

進(jìn)入with語句，調(diào)用__enter__补疑；退出with語句歧沪，調(diào)用__exit__

事實(shí)上sample并不是sample=Sample()，而是__enter__返回的對(duì)象癣丧，即如果__enter__沒有return槽畔，sample將為None。

exc_type：異常類型胁编；exc_val：異常值厢钧；exc_tb：traceback。如果with語句中有異常發(fā)生嬉橙，__exit__中會(huì)收集這些異常信息早直。

async with

async with的用法和with一樣，只是內(nèi)部使用__aenter__和__aexit__來定義上下文市框。這樣我們就能在上下文中使用異步編程霞扬。

class Lock(object):
    def __init__(self, redis_client, key="", ttl=60, timeout=30, interval=0.03, is_wait=True):
        """
        :param key:
        :param ttl: expire time(鎖最大過期時(shí)間) / s
        :param timeout: timeout(阻塞最大時(shí)間) / s
        :param interval: sleep interval(睡眠間隔) / s
        :return:
        Usage::

            with Lock('my_lock') as lock:
                pass
        """
        self.redis_client = redis_client
        self.key = f"redis:lock:{key}"
        self.ttl = ttl
        self.timeout = timeout
        self.interval = interval
        self.value = uuid.uuid1().hex
        self.is_wait = is_wait

    async def __aenter__(self):
        res, wait_times = await self.async_acquire()
        return wait_times

    async def __aexit__(self, exc_type, exc, tb):
        self.release()

    async def async_acquire(self):
        timeout = self.timeout
        wait_times = 0  # 嘗試獲取鎖的次數(shù)
        if timeout == 0:
            while True:
                wait_times += 1
                # redis操作理論上也需要改為異步，由于redis操作很快枫振，暫時(shí)先用同步
                if self.redis_client.set(self.key, self.value, ex=self.ttl, nx=True):
                    return True, wait_times
                if not self.is_wait:  # 不等待鎖
                    raise TooFrequentException('Operation is too frequent')
                await asyncio.sleep(self.interval)
        else:
            while timeout >= 0:
                wait_times += 1
                if self.redis_client.set(self.key, self.value, ex=self.ttl, nx=True):
                    return True, wait_times
                timeout -= self.interval
                if not self.is_wait:  # 不等待鎖
                    raise TooFrequentException('Operation is too frequent')
                await asyncio.sleep(self.interval)
            raise LockTimeout("Timeout whilst waiting for lock")