使用aiohttp嵌套“与...同步”

Question

I would like to create a scheduler class that uses aiohttp to make API calls. I tried this:

import asyncio
import aiohttp

class MySession:
    def __init__(self):
        self.session = None

    async def __aenter__(self):
        async with aiohttp.ClientSession() as session:
            self.session = session
            return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        if self.session:
            await self.session.close()

async def method1():
    async with MySession() as s:
        async with s.session.get("https://www.google.com") as resp:
            if resp.status == 200:
                print("successful call!")

loop = asyncio.get_event_loop()
loop.run_until_complete(method1())
loop.close()

but this just results in an error: RuntimeError: Session is closed . A second approach for the __aenter__ function:

    async def __aenter__(self):
        self.session = aiohttp.ClientSession()
        return self

works well. Is this a good construct? It doesn't adhere to examples of how to use aiohttp. Also wondering why the first approach isn't working?

Answer 1

You can't use with inside a function and have the context manager remain open, no. The with with aiohttp.ClientSession() as session: block exits as soon as you use return to exit the __aenter__ coroutine!

For the specific case, entering a aiohttp.ClientSession() context manager does nothing but return self . So for that type , just creating the instance and storing it in self.session , and awaiting on self.session.close() suffices here, yes.

The general pattern for a nested asynchronous context manager is to await the __aenter__ and __aexit__ methods of a nested async context manager from your own such methods (and perhaps pass along the exception information):

class MySession:
    def __init__(self):
        self.session = None

    async def __aenter__(self):
        self.session = aiohttp.ClientSession()
        await self.session.__aenter__()
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        if self.session:
            return await self.session.__aexit__(exc_type, exc_val, exc_tb)

Technically speaking, you should first assure that there is an actual __aexit__ attribute before entering a nested context manager:

class MySession:
    def __init__(self):
        self.session = None
        self._session_aexit = None

    async def __aenter__(self):
        self.session = aiohttp.ClientSession()
        self._session_aexit = type(self.session).__aexit__
        await self.session.__aenter__()
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        if self.session:
            return await self._session_aexit.__aexit__(
                self.session, exc_type, exc_val, exc_tb)

See the official PEP that added the concept .

Answer 2

You can manage that dependency externally:

import asyncio
import aiohttp

class MySession:

    def __init__(self, client_session):
        self.session = client_session

    async def __aenter__(self):
        return self

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        pass

async def method1():
    async with aiohttp.ClientSession() as client_session:
        async with MySession(client_session) as s:
            async with s.session.get("https://www.google.com") as resp:
                if resp.status == 200:
                    print("successful call!")

asyncio.run(method1())

When that async with chain becomes too ridiculous you can use AsyncExitStack :

from contextlib import AsyncExitStack

async def method1():
    async with AsyncExitStack() as stack:
        client_session = await stack.enter_async_context(aiohttp.ClientSession())
        s = await stack.enter_async_context(MySession(client_session))
        async with s.session.get("https://www.google.com") as resp:
            if resp.status == 200:
                print("successful call!")