Asyncio.create_subprocess_exec NotImplementedError - Fastapi Background Task

Question

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 using an asyncio loop, not uvloop.

import asyncio
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 fix this?

Thanks!

What are you using for your event loop? FastAPI doesn't ship with its own, and the problem you're seeing is due to using an event loop that inherits from `asyncio`'s `BaseEventLoop` without providing an implementation of `_make_subprocess_transport` (the concrete classes `ProactorEventLoop` and `SelectorEventLoop` both define it, and they're the default on Windows and UNIX-likes respectively). What are you using, and how are you using it (basically, can you provide a [MCVE])? — ShadowRanger, Feb 05 '20 at 21:34
I am running this in windows using an asyncio event loop and not uvloop. I have updated the code above to provide a minimal reproducible example. — Bshaw, Feb 06 '20 at 13:48

Hedde van der Heide · Answer 1 · 2020-02-05T20:25:35.067

0

I believe this error is triggered because fastapi uses uvloop and asyncio doesn't know about this without setting a policy, there's an answer that provides some hooks how to achieve this;

asyncio event loop equivalent with uvloop

edited Feb 05 '20 at 20:25

answered Feb 05 '20 at 19:24

Hedde van der Heide

21,841
13
71
100

thank you, however I am running this on windows using an asyncio event loop. – Bshaw Feb 06 '20 at 14:00

Asyncio.create_subprocess_exec NotImplementedError - Fastapi Background Task

1 Answers1

Linked