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Python 3.4中的“异步”

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aiohttp的入门文档提供了以下客户端示例：

import asyncio
import aiohttp

async def fetch_page(session, url):
    with aiohttp.Timeout(10):
        async with session.get(url) as response:
            assert response.status == 200
            return await response.read()

loop = asyncio.get_event_loop()
with aiohttp.ClientSession(loop=loop) as session:
    content = loop.run_until_complete(
        fetch_page(session, 'http://python.org'))
    print(content)

他们为Python 3.4用户提供以下注释：

如果您使用的是Python 3.4，请使用@coroutine装饰器替换等于yield和async def .

如果我遵循这些说明，我会得到：

import aiohttp
import asyncio

@asyncio.coroutine
def fetch(session, url):
    with aiohttp.Timeout(10):
        async with session.get(url) as response:
            return (yield from response.text())

if __name__ == '__main__':
    loop = asyncio.get_event_loop()
    with aiohttp.ClientSession(loop=loop) as session:
        html = loop.run_until_complete(
            fetch(session, 'http://python.org'))
        print(html)

但是，这不会运行，因为Python 3.4不支持 async with ：

$ python3 client.py 
  File "client.py", line 7
    async with session.get(url) as response:
             ^
SyntaxError: invalid syntax

如何将 async with 语句翻译为与Python 3.4一起使用？

python python-3.x async-await python-asyncio aiohttp

2 回答

  • 17

    aiohttp 的examples使用3.4语法实现 . 基于json client example，您的功能将是：

    @asyncio.coroutine
def fetch(session, url):
    with aiohttp.Timeout(10):
        resp = yield from session.get(url)
        try:
            return (yield from resp.text())
        finally:
            yield from resp.release()

    Upd:

    请注意，Martijn的解决方案适用于简单的情况，但在特定情况下可能会导致不必要的行为：

    @asyncio.coroutine
def fetch(session, url):
    with aiohttp.Timeout(5):
        response = yield from session.get(url)

        # Any actions that may lead to error:
        1/0

        return (yield from response.text())

# exception + warning "Unclosed response"

    除了例外你'll get also warning 2440837 . This may lead to connections leak in complex app. You will avoid this problem if you' ll手动调用 resp.release() / resp.close() ：

    @asyncio.coroutine
def fetch(session, url):
    with aiohttp.Timeout(5):
        resp = yield from session.get(url)
        try:

            # Any actions that may lead to error:
            1/0

            return (yield from resp.text())
        except Exception as e:
            # .close() on exception.
            resp.close()
            raise e
        finally:
            # .release() otherwise to return connection into free connection pool.
            # It's ok to release closed response:
            # https://github.com/KeepSafe/aiohttp/blob/master/aiohttp/client_reqrep.py#L664
            yield from resp.release()

# exception only

    我认为最好遵循官方示例（和 __aexit__ implementation）并明确地调用 resp.release() / resp.close() .

    回复于
  • 5

    只是不要使用 session.get() 的结果作为上下文管理器;直接用它作为协程 . session.get() 生成的请求上下文管理器通常在退出时release the request，但是so does using response.text()，所以你可以在这里忽略它：

    @asyncio.coroutine
def fetch(session, url):
    with aiohttp.Timeout(10):
        response = yield from session.get(url)
        return (yield from response.text())

    这里返回的请求包装器没有必需的异步方法（ __aenter__ 和 __aexit__ ），它们在不使用Python 3.5时完全省略（参见relevant source code） .

    如果 session.get() 调用和访问 response.text() 之间有更多的语句，你可能想要使用 try:..finally: 来释放连接;如果发生异常，Python 3.5发行版上下文管理器也会关闭响应 . 因为这里需要 yield from response.release() ，所以在Python 3.4之前不能将它封装在上下文管理器中：

    import sys

@asyncio.coroutine
def fetch(session, url):
    with aiohttp.Timeout(10):
        response = yield from session.get(url)
        try:
            # other statements
            return (yield from response.text())
        finally:
            if sys.exc_info()[0] is not None:
                # on exceptions, close the connection altogether
                response.close()
            else:
                yield from response.release()
    回复于

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