Python module 'asyncio' has no attribute 'to_thread'

Question

From python's asyncio examples:

import asyncio
import time
def blocking_io():
    print(f"start blocking_io at {time.strftime('%X')}")
    # Note that time.sleep() can be replaced with any blocking
    # IO-bound operation, such as file operations.
    time.sleep(1)
    print(f"blocking_io complete at {time.strftime('%X')}")

async def main():
    print(f"started main at {time.strftime('%X')}")

    await asyncio.gather(
        asyncio.to_thread(blocking_io),
        asyncio.sleep(1))

    print(f"finished main at {time.strftime('%X')}")


asyncio.run(main())

# Expected output:
#
# started main at 19:50:53
# start blocking_io at 19:50:53
# blocking_io complete at 19:50:54
# finished main at 19:50:54

It is outputting the next error:

    asyncio.to_thread(blocking_io),
    AttributeError: module 'asyncio' has no attribute 'to_thread'

Has this feature been deprecated? What would be an alternative for threading with asyncio?

to_thread was added in 3.9. For older versions use run_in_executor. — dirn
– dirn, Commented Jul 26, 2021 at 3:50

Bechma · Accepted Answer · 2021-09-13 15:41:31Z

23

to_thread is only available in python 3.9+, if you are working with python 3.8 or an older version, you can copy the source code of it:

async def to_thread(func, /, *args, **kwargs):
    loop = asyncio.get_running_loop()
    ctx = contextvars.copy_context()
    func_call = functools.partial(ctx.run, func, *args, **kwargs)
    return await loop.run_in_executor(None, func_call)

This method copies the context to the thread(to use the current value of your set ContextVars)

If you don't need that and just want a one liner:

await asyncio.get_running_loop().run_in_executor(None, blocking_io, arg1, arg2)

More info about run_in_executor

answered Sep 13, 2021 at 15:41

Bechma

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Comments

Troy Sincomb · Accepted Answer · 2021-12-02 22:00:34Z

4

The top answer didn't help me since was already using python 3.9.7.and continued to get the error

File "<conda_path>/python3.9/site-packages/starlette/concurrency.py", line 39, in run_in_threadpool
    return await anyio.to_thread.run_sync(func, *args)
AttributeError: module 'anyio' has no attribute 'to_thread'

I found the issue in my pypi installs where a package was not updating and needed a force update.

pip install --upgrade --force-reinstall -r requirements.txt

My requirements.txt contained these libraries so it could be anyone of them that was reinstalled and fixed my problem.

moto
uvicorn
fastapi
gunicorn
dnspython

answered Dec 2, 2021 at 22:00

Troy Sincomb

761 silver badge4 bronze badges

1 Comment

Frankie Drake Over a year ago

Updating gunicorn and uvicorn lbraries helped! Thank you

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