Cannot import name 'checkpoint_if_cancelled' from partially initialized module 'anyio.lowlevel'

[CNSeniorious000]

I happened to discover this issue when using `mcp` CLI, which is weird:

> mcp

Exception in thread hmr-daemon:
Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "D:\dev\m\.venv\Scripts\mcp.exe\__main__.py", line 4, in <module>
    from mcp.cli import app
  File "D:\dev\m\.venv\Lib\site-packages\mcp\__init__.py", line 1, in <module>
    from .client.session import ClientSession
  File "D:\dev\m\.venv\Lib\site-packages\mcp\client\session.py", line 5, in <module>
    import anyio.lowlevel
  File "D:\dev\m\.venv\Lib\site-packages\anyio\lowlevel.py", line 9, in <module>
    from .abc import AsyncBackend
  File "D:\dev\m\.venv\Lib\site-packages\anyio\abc\__init__.py", line 43, in <module>
    from .._core._synchronization import (
    ...<5 lines>...
    )
  File "D:\dev\m\.venv\Lib\site-packages\anyio\_core\_synchronization.py", line 12, in <module>
    from ..lowlevel import checkpoint_if_cancelled
ImportError: cannot import name 'checkpoint_if_cancelled' from partially initialized module 'anyio.lowlevel' (most likely due to a circular import) (D:\dev\m\.venv\Lib\site-packages\anyio\lowlevel.py)

Here is a minimal reproducible script:

from time import sleep

sleep(0.01)

import anyio.lowlevel  # noqa: E402, F401

Traceback:

> python bug.py

Exception in thread hmr-daemon:
Traceback (most recent call last):
  File "D:\dev\m\bug.py", line 5, in <module>
    import anyio.lowlevel  # noqa: E402, F401
    ^^^^^^^^^^^^^^^^^^^^^
  File "D:\dev\m\.venv\Lib\site-packages\anyio\lowlevel.py", line 9, in <module>
    from .abc import AsyncBackend
  File "D:\dev\m\.venv\Lib\site-packages\anyio\abc\__init__.py", line 43, in <module>
    from .._core._synchronization import (
    ...<5 lines>...
    )
  File "D:\dev\m\.venv\Lib\site-packages\anyio\_core\_synchronization.py", line 12, in <module>
    from ..lowlevel import checkpoint_if_cancelled
ImportError: cannot import name 'checkpoint_if_cancelled' from partially initialized module 'anyio.lowlevel' (most likely due to a circular import) (D:\dev\m\.venv\Lib\site-packages\anyio\lowlevel.py)

[CNSeniorious000]

Root Cause Analysis

This is a race condition in anyio's get_async_backend() function during concurrent module imports. While Python's import system has locks for individual modules, anyio's global loaded_backends dictionary lacks thread synchronization.

Why Race Condition Occurs

# anyio/_core/_eventloop.py
def get_async_backend(asynclib_name):
    try:
        return loaded_backends[asynclib_name]  # Thread-unsafe check
    except KeyError:
        module = import_module(f"anyio._backends._{asynclib_name}")  # Thread-safe
        loaded_backends[asynclib_name] = module.backend_class  # Thread-unsafe update
        return module.backend_class

Multiple threads can simultaneously:

1. Check loaded_backends → KeyError
2. Execute import_module() (protected by import lock)
3. Update loaded_backends without synchronization

Solution Implemented

Added thread-safe monkey patch in hmr-daemon:

_anyio_lock = Lock()
@wraps(original_get_async_backend)
def thread_safe_get_async_backend(asynclib_name=None):
    with _anyio_lock:
        return original_get_async_backend(asynclib_name)

Related Issues

• anyio#210 - Race condition in get_asynclib
• httpcore#395 - Similar "partially initialized module" errors

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