「異步與」在Python 3.4

Question

的獲取對於aiohttp入門文檔給下面的客戶端的例子：

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) 

，他們給的Python 3.4用戶提供了以下注意事項：

如果您正在使用Python 3.4，請使用@coroutine裝飾器替換await和 async def。

如果我按照這些指示，我得到：

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，這將無法運行，因爲async with沒有在Python 3.4的支持：

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

如何翻譯的async with語句與Python 3.4一起工作？

Answer 1

只要不使用session.get()的結果作爲上下文管理器;直接使用它作爲協程。請求上下文管理器session.get()產生通常會release the request退出，但so does using response.text()，所以你可以忽略這裏：

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

包裝這裏返回的請求不具有所需的異步方法（__aenter__和__aexit__），他們在不使用Python 3.5時完全省略（請參閱relevant source code）。

如果您在session.get()調用和訪問response.text()之間有更多的語句可用，您可能想要使用try:..finally:來釋放連接;如果發生異常，Python 3.5版本上下文管理器還會關閉響應。由於yield from response.release()這裏需要，這不能在上下文管理器封裝的Python 3.4之前：

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使用3.4語法來實現。基於json client example你的函數是：

@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：

注意的Martijn的解決方案將工作簡單案件，但可能導致在特定情況下不必要的行爲：

@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，您還會收到警告「未公開回復」。這可能會導致複雜應用程序中的連接泄漏。你會避免這個問題，如果你手動調用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 

我覺得這是更好地遵循正式的例子（和__aexit__implementation）和呼叫resp.release()/resp.close()明確。