uvicorn can not start FastAPI with example settings

[cgi1]

import uvicorn
from fastapi import FastAPI
app = FastAPI(title='MADS API')
uvicorn.run(app, host='0.0.0.0', port=8127, workers=2)
WARNING:  You must pass the application as an import string to enable 'reload' or 'workers'.

than it quits with SystemError 1

Corresponding ticket in uvicorn

Versions:

$ pip3 list |grep "uvicorn\|fastapi"
fastapi                          0.55.1       
uvicorn                          0.11.5 

[jorgerpo]

Why do you say it is a bug?
It is your first try at FastAPI?

[cgi1]

No, i upgraded from Ubuntu 18.04 to Ubuntu 20.04. It was running fine on Ubuntu 18.04. productive a long time like this. So it looks like the interfaces were changing?

[phy25]

You must pass the application as an import string to enable 'reload' or 'workers'.

Is the program stopping here? Looks like it's only a warning but it's very straightforward about what's going on.

[cgi1]

@phy25 Yes it stops here. I was searching through a bit, and found this issue report on uvicorn under Windows.

[cgi1]

When I try it as supposed in the related issues, it gives me errors as well:

uvicorn.run("app", host='0.0.0.0', port=8127, workers=2)
ERROR:    Error loading ASGI app. Import string "app" must be in format "<module>:<attribute>".

and

uvicorn.run("main:app", host='0.0.0.0', port=8127, workers=2)
ERROR:    Error loading ASGI app. Could not import module "main".

So uvicorn did not found the FastAPI application.

[phy25]

This is intended and not related to fastapi:

https://github.com/encode/uvicorn/blob/9b92925a352b9743c5cfcef4a65e74a81a1bad4f/uvicorn/main.py#L343

You need to run it with command line uvicorn if you want to run with multiple workers. Please read uvicorn documentation.

[tiangolo]

Thanks for the help here @phy25 ! 👏 🙇

If that solves the original problem, then you can close this issue @cgi1 ✔️

[cgi1]

The answer I was searching for is here: You can actually start it the way I wanted it, but make sure to name it correctly:

uvicorn.run("main:app", host='0.0.0.0', port=8127, workers=2)
ERROR:    Error loading ASGI app. Could not import module "main".

raises Error because the file is not named main.py! If you have it as my_fastapi_server.py, you run it as follows:

uvicorn.run("my_fastapi_server:app", host='0.0.0.0', port=8127, workers=2)

[mrtolkien]

Adding another snippet as the ones posted here were pretty cryptic for me.

With the following files:

app\main.py    # Containing an app object
debug_server.py

I needed my debug_server.py to be:

import uvicorn

# Importing app here makes the syntax cleaner as it will be picked up by refactors
from app.main import app

if __name__ == "__main__":
    uvicorn.run("debug_server:app", host="0.0.0.0", port=80, reload=True)

That way the debug auto-reload server did work properly and there’s nothing related to it in the /app folder.

[chandresholaniya]

I got following error

$ uvicorn main:app --reload
←[32mINFO←[0m: Uvicorn running on ←[1mhttp://127.0.0.1:8000←[0m (Press CTRL+C to quit)
←[32mINFO←[0m: Started reloader process [←[36m←[1m22984←[0m] using ←[36m←[1mwatchgod←[0m
←[31mERROR←[0m: Error loading ASGI app. Could not import module "main".
←[32mINFO←[0m: Stopping reloader process [←[36m←[1m22984←[0m]

I am using uvicorn [0.13.4] on Windows 10

Kindly suggest steps to resolve it.

[waketzheng]

The answer I was searching for is here: You can actually start it the way I wanted it, but make sure to name it correctly:

uvicorn.run("main:app", host='0.0.0.0', port=8127, workers=2)
ERROR:    Error loading ASGI app. Could not import module "main".

raises Error because the file is not named main.py! If you have it as my_fastapi_server.py, you run it as follows:

uvicorn.run("my_fastapi_server:app", host='0.0.0.0', port=8127, workers=2)

You can use pathlib to avoid this problem:

from pathlib imprt Path

uvicorn.run(f"{Path(__file__).stem}:app", host='0.0.0.0', port=8127, workers=2)

[timhughes]

Try this.

import uvicorn
from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def get_root():
    return {"message": "Hello World"}


if __name__ == "__main__":
    uvicorn.run("__main__:app", host="0.0.0.0", port=8000, reload=True, workers=2)

[rafaelcascalho]

What also worked for me was this:

inside the api.py file

if __name__ == "__main__":
    run(app="api:app", reload=True, port=8080, host="0.0.0.0")

[timhughes]

What also worked for me was this:

inside the api.py file

if __name__ == "__main__":
    run(app="api:app", reload=True, port=8080, host="0.0.0.0")

This depends on your file being named api.py. Using __main__.app means you can rename your app.py file to anything and it will still work.

Further reading https://docs.python.org/3/library/__main__.html

[tiangolo]

Thanks for reporting back and closing the issue @cgi1 ! 🍰

And thanks everyone else for the discussions here. ☕