"async with" 在 Python 3.4

Question

aiohttp 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：

If you are using Python 3.4, please replace await with yield from and async def with a @coroutine decorator.

如果我按照这些说明操作，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)

然而，这不会运行，因为async with在Python 3.4中不被支持：

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

我如何翻译 async with 语句以使用 Python 3.4？

Answer 1

只是不要将 session.get() 的结果用作上下文管理器；直接将其用作协程。 session.get() 生成的请求上下文管理器通常会 release the request on exit, but 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())

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

如果在 session.get() 调用和访问 response.text() 等待之间有更多的语句，您可能想要使用 try:..finally: 来释放连接；如果发生异常，Python 3.5 版上下文管理器还会关闭响应。因为这里需要一个yield from response.release()，所以在Python 3.4:

之前，这个不能封装在context manager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()

Answer 2

aiohttp 的 examples implemented using 3.4 syntax. Based on json client example 您的函数将是：

@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()

更新：

请注意，Martijn 的解决方案适用于简单情况，但在特定情况下可能会导致不良行为：

@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"

除了异常你还会收到警告"Unclosed response"。这可能会导致复杂应用程序中的连接泄漏。如果您手动调用 resp.release()/resp.close():

就可以避免这个问题

@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）并显式调用resp.release()/resp.close()。

"async with" 在 Python 3.4

"async with" in Python 3.4

python

python-3.x

async-await

python-asyncio

aiohttp