python - 如何管理单个aiohttp.ClientSession？

作为一项学习练习，我正在尝试修改aiohttp的快速入门示例，以使用单个ClientSession来获取多个URL(文档建议通常应为每个应用程序创建一个ClientSession)。

import aiohttp
import asyncio

async def fetch(session, url):
  async with session.get(url) as response:
    return await response.text()

async def main(url, session):
  print(f"Starting '{url}'")
  html = await fetch(session, url)
  print(f"'{url}' done")

urls = (
  "https://python.org",
  "https://twitter.com",
  "https://tumblr.com",
  "https://example.com",
  "https://github.com",
)

loop = asyncio.get_event_loop()
session = aiohttp.ClientSession()
loop.run_until_complete(asyncio.gather(
  *(loop.create_task(main(url, session)) for url in urls)
))
# session.close()   <- this doesn't make a difference

但是，在协程之外创建ClientSession显然不是可行的方法:

➜ python 1_async.py
1_async.py:30: UserWarning: Creating a client session outside of coroutine is a very dangerous idea
  session = aiohttp.ClientSession()
Creating a client session outside of coroutine
client_session:
Starting 'https://python.org'
Starting 'https://twitter.com'
Starting 'https://tumblr.com'
Starting 'https://example.com'
Starting 'https://github.com'
'https://twitter.com' done
'https://example.com' done
'https://github.com' done
'https://python.org' done
'https://tumblr.com' done
1_async.py:34: RuntimeWarning: coroutine 'ClientSession.close' was never awaited
  session.close()
Unclosed client session
client_session:
Unclosed connector
connections: ['[(, 15024.110107067)]', '[(, 15024.147785039)]', '[(, 15024.252375415)]', '[(, 15024.292646968)]', '[(, 15024.342368087)]', '[(, 15024.466971983)]', '[(, 15024.602057745)]', '[(, 15024.837045568)]']
connector:

FWIW, this was main before I attempted the above change:

async def main(url):
  async with aiohttp.ClientSession() as session:
    print(f"Starting '{url}'")
    html = await fetch(session, url)
    print(f"'{url}' done")

正确的方法是什么？我曾考虑过将URL列表传递给main，但无法使其以非顺序的方式工作。

最佳答案

Creating a client session outside of coroutine is a very dangerous idea的原因在于，当您创建它时，它将绑定(bind)到当前循环。如果之后更改运行循环，它将挂起。但是，如果您足够仔细地使用它，则可以忽略它。 Related doc。

对于我来说，我只是不理会这个警告。但是也很容易克服它:

async def create_session():
    return aiohttp.ClientSession()

session = asyncio.get_event_loop().run_until_complete(create_session())

此外，您无需显式创建Task对象，而只需执行此协程函数:

loop.run_until_complete(asyncio.gather(
  *(main(url, session) for url in urls)
))

最后，别忘了close是协程。您应该使用loop.run_until_complete(session.close())关闭session。

顺便说一句，如果您想创建一个类似异步的循环，可以引用my another answer。

关于python - 如何管理单个aiohttp.ClientSession？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/51908915/