Issue
I'm trying to run a Subprocess in a FastApi route, but the execution results in NotImplementedError. I've read similar questions on the issue:
Why am I getting NotImplementedError with async and await on Windows?
Asyncio.create_subprocess_exec NotImplementedError - Fastapi Background Task
But it doesn't seem they have any viable solutions.
My FastApi route looks like this:
@app.get("/test/subprocess")
async def subprocess_test():
parsed_json = await(probe_video_file(Path(r"G:\ffmpeg testing\ffmpeg\ffprobe.exe"),
Path(r"G:\ffmpeg testing\input_file\test_file.wmv")))
print(parsed_json)
return parsed_json
When I navigate to this rout, an exception is raised and the code crushes.
The probe_video_file function has a subprocess call inside it, like so:
async def probe_video_file(ffprobe_path: Path, file_to_probe: Path) -> dict:
"""
Probes a video file with FFprobe.
:param ffprobe_path: Path to ffprobe executable
:param file_to_probe: Path to file to probe
:returns Parsed JSON dict of the output
"""
args = [f'{str(ffprobe_path)}', f'{str(file_to_probe)}']
args += ["-hide_banner", "-loglevel", "fatal", "-show_error", "-show_format", "-show_streams", "-show_chapters",
"-show_private_data", "-print_format", "json"]
#sub process is here, and here is where the exception happens.
proc = await asyncio.create_subprocess_exec(
*args,
stdout=asyncio.subprocess.PIPE,
stderr=asyncio.subprocess.PIPE)
stdout, stderr = await proc.communicate()
print(f'exited with {proc.returncode}]')
if stdout:
print(f'[stdout]\n{stdout.decode()}')
if stderr:
print(f'[stderr]\n{stderr.decode()}')
raise Exception(f"Failed to probe file {file_to_probe}")
return json.loads(stdout)
When I run probe_video_file without FastAPI, like so:
async def main():
json_output = await probe_video_file(Path(r"G:\ffmpeg testing\ffmpeg\ffprobe.exe"),
Path(r"G:\ffmpeg testing\input_file\test_file.wmv"))
print(json_output)
if __name__ == '__main__':
asyncio.run(main())
It runs just fine and prints the correct output:
[['G:\\ffmpeg testing\\ffmpeg\\ffprobe.exe', 'G:\\ffmpeg testing\\input_file\\test_file.wmv', '-hide_banner', '-loglevel', 'fatal', '-show_error', '-show_format', '-show_streams', '-show_chapters', '-show_private_data', '-print_format', 'json'] exited with 0]
[stdout]
{
"streams": [
{
"index": 0,
"codec_name": "wmav2",
"codec_long_name": "Windows Media Audio 2",
"codec_type": "audio",
"codec_tag_string": "a[1][0][0]",
"codec_tag": "0x0161",
"sample_fmt": "fltp",
"sample_rate": "48000",
"channels": 2,
"bits_per_sample": 0,
"r_frame_rate": "0/0",
"avg_frame_rate": "0/0",
"time_base": "1/1000",
"start_pts": 0,
"start_time": "0.000000",
"duration_ts": 2155050,
"duration": "2155.050000",
"bit_rate": "96000",
"disposition": {
"default": 0,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0
},
"tags": {
"language": "eng"
}
},
{
"index": 1,
"codec_name": "wmv3",
"codec_long_name": "Windows Media Video 9",
"profile": "Main",
"codec_type": "video",
"codec_tag_string": "WMV3",
"codec_tag": "0x33564d57",
"width": 850,
"height": 480,
"coded_width": 850,
"coded_height": 480,
"closed_captions": 0,
"has_b_frames": 0,
"pix_fmt": "yuv420p",
"level": -99,
"chroma_location": "left",
"refs": 1,
"r_frame_rate": "30000/1001",
"avg_frame_rate": "30000/1001",
"time_base": "1/1000",
"start_pts": 0,
"start_time": "0.000000",
"duration_ts": 2155050,
"duration": "2155.050000",
"bit_rate": "2000000",
"disposition": {
"default": 0,
"dub": 0,
"original": 0,
"comment": 0,
"lyrics": 0,
"karaoke": 0,
"forced": 0,
"hearing_impaired": 0,
"visual_impaired": 0,
"clean_effects": 0,
"attached_pic": 0,
"timed_thumbnails": 0
},
"tags": {
"language": "eng"
}
}
],
"chapters": [
],
"format": {
"filename": "G:\\ffmpeg testing\\input_file\\test_file.wmv",
"nb_streams": 2,
"nb_programs": 0,
"format_name": "asf",
"format_long_name": "ASF (Advanced / Active Streaming Format)",
"start_time": "0.000000",
"duration": "2155.050000",
"size": "567194391",
"bit_rate": "2105545",
"probe_score": 100,
"tags": {
"WMFSDKNeeded": "0.0.0.0000",
"DeviceConformanceTemplate": "MP@HL",
"WMFSDKVersion": "11.0.5721.5265",
"IsVBR": "0"
}
}
}
{'streams': [{'index': 0, 'codec_name': 'wmav2', 'codec_long_name': 'Windows Media Audio 2', 'codec_type': 'audio', 'codec_tag_string': 'a[1][0][0]', 'codec_tag': '0x0161', 'sample_fmt': 'fltp', 'sample_rate': '48000', 'channels': 2, 'bits_per_sample': 0, 'r_frame_rate': '0/0', 'avg_frame_rate': '0/0', 'time_base': '1/1000', 'start_pts': 0, 'start_time': '0.000000', 'duration_ts': 2155050, 'duration': '2155.050000', 'bit_rate': '96000', 'disposition': {'default': 0, 'dub': 0, 'original': 0, 'comment': 0, 'lyrics': 0, 'karaoke': 0, 'forced': 0, 'hearing_impaired': 0, 'visual_impaired': 0, 'clean_effects': 0, 'attached_pic': 0, 'timed_thumbnails': 0}, 'tags': {'language': 'eng'}}, {'index': 1, 'codec_name': 'wmv3', 'codec_long_name': 'Windows Media Video 9', 'profile': 'Main', 'codec_type': 'video', 'codec_tag_string': 'WMV3', 'codec_tag': '0x33564d57', 'width': 850, 'height': 480, 'coded_width': 850, 'coded_height': 480, 'closed_captions': 0, 'has_b_frames': 0, 'pix_fmt': 'yuv420p', 'level': -99, 'chroma_location': 'left', 'refs': 1, 'r_frame_rate': '30000/1001', 'avg_frame_rate': '30000/1001', 'time_base': '1/1000', 'start_pts': 0, 'start_time': '0.000000', 'duration_ts': 2155050, 'duration': '2155.050000', 'bit_rate': '2000000', 'disposition': {'default': 0, 'dub': 0, 'original': 0, 'comment': 0, 'lyrics': 0, 'karaoke': 0, 'forced': 0, 'hearing_impaired': 0, 'visual_impaired': 0, 'clean_effects': 0, 'attached_pic': 0, 'timed_thumbnails': 0}, 'tags': {'language': 'eng'}}], 'chapters': [], 'format': {'filename': 'G:\\ffmpeg testing\\input_file\\test_file.wmv', 'nb_streams': 2, 'nb_programs': 0, 'format_name': 'asf', 'format_long_name': 'ASF (Advanced / Active Streaming Format)', 'start_time': '0.000000', 'duration': '2155.050000', 'size': '567194391', 'bit_rate': '2105545', 'probe_score': 100, 'tags': {'WMFSDKNeeded': '0.0.0.0000', 'DeviceConformanceTemplate': 'MP@HL', 'WMFSDKVersion': '11.0.5721.5265', 'IsVBR': '0'}}}
Process finished with exit code 0
I've tried 'setting' the event loop to ProactorEventLoop like it's suggested in the other questions:
@app.on_event("startup")
async def startup_event():
"""Code runs at startup..."""
loop = asyncio.ProactorEventLoop()
asyncio.set_event_loop(loop)
But it had no effect.
Here is the exception traceback:
ERROR: Exception in ASGI application
Traceback (most recent call last):
File "E:\pycharm\my_project\venv\lib\site-packages\uvicorn\protocols\http\httptools_impl.py", line 375, in run_asgi
result = await app(self.scope, self.receive, self.send)
File "E:\pycharm\my_project\venv\lib\site-packages\uvicorn\middleware\proxy_headers.py", line 75, in __call__
return await self.app(scope, receive, send)
File "E:\pycharm\my_project\venv\lib\site-packages\fastapi\applications.py", line 208, in __call__
await super().__call__(scope, receive, send)
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\applications.py", line 112, in __call__
await self.middleware_stack(scope, receive, send)
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\middleware\errors.py", line 181, in __call__
raise exc
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\middleware\errors.py", line 159, in __call__
await self.app(scope, receive, _send)
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\middleware\cors.py", line 84, in __call__
await self.app(scope, receive, send)
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\exceptions.py", line 82, in __call__
raise exc
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\exceptions.py", line 71, in __call__
await self.app(scope, receive, sender)
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\routing.py", line 656, in __call__
await route.handle(scope, receive, send)
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\routing.py", line 259, in handle
await self.app(scope, receive, send)
File "E:\pycharm\my_project\venv\lib\site-packages\starlette\routing.py", line 61, in app
response = await func(request)
File "E:\pycharm\my_project\venv\lib\site-packages\fastapi\routing.py", line 226, in app
raw_response = await run_endpoint_function(
File "E:\pycharm\my_project\venv\lib\site-packages\fastapi\routing.py", line 159, in run_endpoint_function
return await dependant.call(**values)
File "E:\pycharm\my_project\src\backend\app\api\api.py", line 202, in subprocess_test
parsed_json = await(probe_video_file(Path(r"G:\ffmpeg testing\ffmpeg\ffprobe.exe"),
File "E:\pycharm\my_project\src\backend\app\ffmpeg\ffprobe.py", line 26, in probe_video_file
proc = await asyncio.create_subprocess_exec(
File "C:\Users\user\AppData\Local\Programs\Python\Python310\lib\asyncio\subprocess.py", line 218, in create_subprocess_exec
transport, protocol = await loop.subprocess_exec(
File "C:\Users\user\AppData\Local\Programs\Python\Python310\lib\asyncio\base_events.py", line 1652, in subprocess_exec
transport = await self._make_subprocess_transport(
File "C:\Users\user\AppData\Local\Programs\Python\Python310\lib\asyncio\base_events.py", line 493, in _make_subprocess_transport
raise NotImplementedError
NotImplementedError
INFO: 127.0.0.1:58576 - "GET /test/subprocess HTTP/1.1" 500 Internal Server Error
Does anyone know how to resolve it? From what I understand this has to do with FastApi's default event loop not supporting this. But I don't know how to set or replace the default FastApi event loop with Asyncio's default event loop.
EDIT:
I think it's important to point out that I'm using windows 10.
Solution
Looking at the uvicorn code a bit closer, it seems that the reload=True options causes the default ProactorEventLoop to be changed to SelectorEventLoop on windows. You can try it without reload=True and see if it works (I can't test it right now). But if you need to enforce the use of a specific event loop in uvicorn, you can subclass uvicorn.Server like so:
import asyncio
from asyncio.windows_events import ProactorEventLoop
from fastapi import FastAPI
from uvicorn import Config, Server
app = FastAPI()
class ProactorServer(Server):
def run(self, sockets=None):
loop = ProactorEventLoop()
asyncio.set_event_loop(loop) # since this is the default in Python 3.10, explicit selection can also be omitted
asyncio.run(self.serve(sockets=sockets))
config = Config(app=app, host="0.0.0.0", port=8000, reload=True)
server = ProactorServer(config=config)
server.run()
This should allow you to use asyncio.create_subprocess_exec() within your FastAPI endpoint.
Answered By - thisisalsomypassword
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