Uvicorn fails "quietly" upon a circular import error

[Kludex]

Discussed in #2034

^{Originally posted by mikeedjones July 8, 2023}
From this comment and discussion.

When there is a circular import in the app, uvicorn reports ERROR: Error loading ASGI app. Could not import module "main". without exposing the stacktrace which would be reported if you ran the code using python main.py. MRE below.

# main.py
from bar import Bar

class Foo:
    pass

async def app(scope, receive, send):
    assert scope['type'] == 'http'

#bar.py
from main import Foo

class Bar:
    pass

Running with

uvicorn main:app

only results in

ERROR:    Error loading ASGI app. Could not import module "main".

whereas running

python main.py

reports the far more useful

ImportError: cannot import name 'Bar' from partially initialized module 'bar' (most likely due to a circular import) (/workspaces/fastapi/bar.py)

Not sure how you'd start debugging this one I'm afraid - or if its even a "bug". For comparison syntax errors are reported correctly.

[mikeedjones]

I'll take a run at this :)

[Kludex]

I'll take a run at this :)

Thanks.

The problem is here:

uvicorn/uvicorn/importer.py

Lines 20 to 26 in 59f0ddb

	try:
	module = importlib.import_module(module_str)
	except ImportError as exc:
	if exc.name != module_str:
	raise exc from None
	message = 'Could not import module "{module_str}".'
	raise ImportFromStringError(message.format(module_str=module_str))

[mikeedjones]

#2038 tries to address this

[Kludex]

• Closed by Raise ImportError on circular import #2040

It will be available on 0.23.0.