Asyncio.create_subprocess_exec from BackgroundTask

[rbraddev]

I am trying to call asyncio.create_subprocess_exec within a Fastapi background task, but it keeps raising a NotImplementedError. The run_subprocess function works fine when it is ran outside of Fastapi. I am running this in windows.

Here is an example code and the error I am getting:

from fastapi import FastAPI, BackgroundTasks

DHCP_SERVER = "1.1.1.1"

app = FastAPI()

@app.get("/")
async def subprocess_test(background_tasks: BackgroundTasks):
  background_tasks.add_task(run_subprocess)

async def run_subprocess():
  proc = await asyncio.create_subprocess_exec(
    'powershell.exe',
    f'Get-Dhcp-Serverv4Scope -ComputerName \"{DHCP_SERVER}\"',
    stdout=asyncio.subprocess.PIPE, stderr=asyncio.subprocess.PIPE
  )

  stdout, stderr = await proc.communicate()

  if stderr:
    print(stderr)
  else:
    print(stdout)


File ".\subprocess_example.py", line 13 in run_subprocess
  proc = await asyncio.create_subprocess_exec(
File "C:\Python\Python38-32\lib\asyncio\subprocess.py", line 236, in create_subprcess_exec
  transport, protocol = await loop.subprocess_exec(
File "C:\Python\Python38-32\lib\asyncio\base_events.py", line 1615, in subprocess_exec
  transport = await self._make_subprocess_transport(
File "C:\Python\Python38-32\lib\asyncio\base_events.py", line 487, in _make_subprocess_transport
  raise NotImplementedError

Can anyone please help?
Thanks

[tiangolo]

Hey @rbraddev , check the note here: https://docs.python.org/3/library/asyncio-eventloop.html#running-subprocesses

Note The default asyncio event loop on Windows does not support subprocesses. See Subprocess Support on Windows for details.

Although the section https://docs.python.org/3/library/asyncio-platforms.html#subprocess-support-on-windows says that the default event loop does support subprocesses...

But maybe you can use a normal def function and run it with subprocess.

[euri10]

Although the section https://docs.python.org/3/library/asyncio-platforms.html#subprocess-support-on-windows says that the default event loop does support subprocesses...

this is the case, ProactorEventLoop does support subprocesses, but since Kludex/uvicorn@abd6870 the auto loop chosen in uvicorn for Windows is not the windows default anymore but asyncio.SelectorEventLoop

[rbraddev]

thanks both. i ended up using celery to run the task.

[github-actions[bot]]

Assuming the original need was handled, this will be automatically closed now. But feel free to add more comments or create new issues or PRs.

[raythurnvoid]

Solution in case you're forced to use SelectorEventLoop: https://stackoverflow.com/a/76981596/1951947