Consider not suppressing the raised exception when loading a factory

[vdwees]

Hi,

Recently came across this error:

Unfortunately its not much to go on. For any non-trivial factory, capturing and suppressing all TypeError makes it hard to debug:

uvicorn/uvicorn/config.py

Lines 470 to 475 in ffa5b1a

	try:
	self.loaded_app = self.loaded_app()
	except TypeError as exc:
	if self.factory:
	logger.error("Error loading ASGI app factory: %s", exc)
	sys.exit(1)

Would it make sense to raise the exception? Or use inspect rather than try-except?

[Kludex]

How would the full stack traceback be helpful here?

[vdwees]

The factory is a real function, and it is getting imported and called correctly. Inside the factory, a module is imported. While parsing some configuration, the module raises a TypeError. It is ambiguous which part of the configuration is incorrect, but a stack trace would make it clear exactly what is going wrong.

[vdwees]

Note that if it had raised any other exception (e.g. KeyError), there would be a stack trace. So uvicorn is treating TypeError as a special case and it suppresses them, because it does not distinguish between TypeErrors raised by the factory and the TypeError raised when attempting to call a non-callable app.

[vdwees]

I'd be happy to submit a PR if that helps clarify.

[mjog]

How would the full stack traceback be helpful here?

The factory is a function, the function throws an error when invoked, and your psychic debugger is not functioning at the moment so you don't know where in the function's code path it is failing.

[vdwees]

It seems to me there are two options:

1. Keep the try/except, but reraise the exception instead of calling sys.exit(1)
2. use inspect to determine if its worth calling loaded_app, and not capture any raised exceptions