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. ☕

[hrishi483]

Try
if name == "main":
uvicorn.run("main:app", host="127.0.0.1", port=8000, reload=True)

• "main:app" is an import string that tells Uvicorn to look for the app object in the main module (i.e., main.py file).
• When reload=True, Uvicorn needs to re-import your application during code changes. Using the import string ensures the re-import works as expected.

This works and fixes the issue

[Norlandz]

Note:
If the project structure is /your_proj/your_package/main.py .
And you are running with python -m your_package.main under your proj dir.

Then using uvicorn.run("your_package.main:app", works.
Or uvicorn.run("__main__:app", works.

---

Que:
Just curious why passing a string is the solution?
I dont think passing string is safer than a instance app?

[timhughes]

You pass an import string so that uvicorn can create the instances. This enables uvicorn to manage the lifecycle of the app and start multiple instances in workers. It also enables the reload functionality.