Broken Compatibility between Curio and AsyncClient

[1ekf]

The documentation lists Curio to be compatible with AsyncClient, but running the sample code raises a ModuleNotFoundError with message No module named 'anyio._backends._curio'. I suspect this is because AnyIO dropped Curio backend support.

Environment

• OS: Arch Linux (x86_64)
• Python version: 3.9.7
• httpx version: 0.21.1
• AnyIO version: 3.4.0
• Curio version: 1.5

Traceback

The following is the traceback on a fresh virtual environment, only with pip install curio httpx. I ran the sample code as given in the httpx documentation via Python interactive mode.

Code

import httpx
import curio

async def main():
    async with httpx.AsyncClient() as client:
        response = await client.get('https://www.example.com/')
        print(response)

curio.run(main)

Traceback

Traceback (most recent call last):
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/anyio/_core/_eventloop.py", line 138, in get_asynclib
    return sys.modules[modulename]
KeyError: 'anyio._backends._curio'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 3, in main
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_client.py", line 1736, in get
    return await self.request(
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_client.py", line 1513, in request
    return await self.send(request, auth=auth, follow_redirects=follow_redirects)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_client.py", line 1600, in send
    response = await self._send_handling_auth(
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_client.py", line 1628, in _send_handling_auth
    response = await self._send_handling_redirects(
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_client.py", line 1665, in _send_handling_redirects
    response = await self._send_single_request(request)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_client.py", line 1702, in _send_single_request
    response = await transport.handle_async_request(request)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_transports/default.py", line 291, in handle_async_request
    resp = await self._pool.handle_async_request(req)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 220, in handle_async_request
    status = RequestStatus(request)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 20, in __init__
    self._connection_acquired = AsyncEvent()
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpcore/_synchronization.py", line 29, in __init__
    self._event = anyio.Event()
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/anyio/_core/_synchronization.py", line 77, in __new__
    return get_asynclib().Event()
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/anyio/_core/_eventloop.py", line 140, in get_asynclib
    return import_module(modulename)
  File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 984, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'anyio._backends._curio'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/anyio/_core/_eventloop.py", line 138, in get_asynclib
    return sys.modules[modulename]
KeyError: 'anyio._backends._curio'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/curio/kernel.py", line 822, in run
    return kernel.run(corofunc, *args)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/curio/kernel.py", line 172, in run
    raise ret_exc
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/curio/kernel.py", line 736, in kernel_run
    trap = current.send(current._trap_result)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/curio/task.py", line 167, in send
    return self._send(value)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/curio/task.py", line 171, in _task_runner
    return await coro
  File "<stdin>", line 4, in main
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_client.py", line 1982, in __aexit__
    await self._transport.__aexit__(exc_type, exc_value, traceback)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpx/_transports/default.py", line 270, in __aexit__
    await self._pool.__aexit__(exc_type, exc_value, traceback)
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 312, in __aexit__
    await self.aclose()
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpcore/_async/connection_pool.py", line 297, in aclose
    async with self._pool_lock:
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/httpcore/_synchronization.py", line 15, in __aenter__
    await self._lock.acquire()
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/anyio/_core/_synchronization.py", line 117, in acquire
    await checkpoint_if_cancelled()
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/anyio/lowlevel.py", line 42, in checkpoint_if_cancelled
    await get_asynclib().checkpoint_if_cancelled()
  File "/home/user/.virtualenvs/curio_httpx/lib/python3.9/site-packages/anyio/_core/_eventloop.py", line 140, in get_asynclib
    return import_module(modulename)
  File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 984, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'anyio._backends._curio'

[tomchristie]

Yup thanks for raising this...

Changed in httpcore 0.14.0 - See the notes here - https://github.com/encode/httpcore/blob/master/CHANGELOG.md#0140-november-11th-2021

Right now it doesn't look like it's worth our time including curio support. (or at the very least it was less important than pushing on with the httpcore redesign, and shortly releasing httpx 1.0)

Anyways you're correct that this is a valid docs issue, and we need to address it.

[Raichuu41]

I've had some random SSL Handshake issues when using asyncio, httpx and httpx-socks. So I wanted to test out something else, tried curio and did a sample test code to test it. However I was getting the same error and found this thread here.

Please update the HTTPX documentation: https://www.python-httpx.org/async/#curio
Should mention there that it doesn't work anymore.