solve_dependencies occasionally raises RuntimeError: generator didn't yield

[Jaza]

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Example Code

import logging
from typing import Annotated, AsyncGenerator

from anyio import open_file
from fastapi import Depends, FastAPI


async def get_thingy() -> AsyncGenerator[str, None]:
    """Should return a thingy, but is a badly behaved dependency, does nothing"""
    try:
        async with await open_file("/path/to/aintnosuchfileyo.txt", "r") as f:
            yield await f.read()
    except Exception:
        logging.warning("Ain't no such file yo!")


app = FastAPI()


@app.get("/")
async def root(thingy: Annotated[str, Depends(get_thingy)]):
    return {"message": f"Hello {thingy}"}

Description

When running a FastAPI app whose endpoints use a number of dependencies (including a python-oracledb async connection pool), the following error occasionally occurs (redacted stack trace copied from production logs):

Traceback (most recent call last):
  ...
  File "/path/to/site-packages/gunicorn/app/base.py", line 235, in run
    super().run()
  ...
  File "/path/to/site-packages/uvicorn/workers.py", line 107, in run
    return asyncio.run(self._serve())
  ...
  File "/path/to/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  ...
  File "/path/to/site-packages/starlette/routing.py", line 73, in app
    response = await f(request)
  File "/path/to/site-packages/fastapi/routing.py", line 291, in app
    solved_result = await solve_dependencies(
  File "/path/to/site-packages/fastapi/dependencies/utils.py", line 615, in solve_dependencies
    solved_result = await solve_dependencies(
  File "/path/to/site-packages/fastapi/dependencies/utils.py", line 634, in solve_dependencies
    solved = await solve_generator(
  File "/path/to/site-packages/fastapi/dependencies/utils.py", line 560, in solve_generator
    return await stack.enter_async_context(cm)
  File "/usr/local/lib/python3.11/contextlib.py", line 638, in enter_async_context
    result = await _enter(cm)
  File "/usr/local/lib/python3.11/contextlib.py", line 206, in __aenter__
    raise RuntimeError("generator didn't yield") from None
RuntimeError: generator didn't yield

This is caused by python-oracledb failing to get a connection from its pool.

Not sure if this is related to the Python core issue Worse error from asynccontextmanager in Python 3.10, which, from a quick Google search, was the only relevant discussion that I could find.

Would be great if FastAPI could catch that RuntimeError: generator didn't yield , and instead raise something more useful, ideally an error message that identifies the dependency responsible for the underlying error. Possibly a custom middleware could catch and re-raise that error too? Not sure, it might need to happen inside solve_dependencies() , where the currently-being-solved dependency is available as a variable.

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.115.6

Pydantic Version

2.10.3

Python Version

3.11.4

Additional Context

No response

[Jaza]

I managed to put together some example code that reliably reproduces this error, I've updated "example code" above.

[luzzodev]

Hello @Jaza,

about the traceback and your error, this is because the generator is no yielding anything due the exception occurring. You are no able to see what exception is because your generator does not re-raise the error.

Please read Dependencies with yield and except.

Hope this helps.

[alv2017]

This is in Technical Details part of the documentation:

Any function that is valid to use with:

@contextlib.contextmanager or @contextlib.asynccontextmanager would be valid to use as a FastAPI dependency.

In fact, FastAPI uses those two decorators internally.

(If you were actually making logs, then you should be able to find the actual (the sensible ones!) error messages in the logs 😀 )

[Jaza]

@luzzodev yes, thanks, I found the section in the docs that you pointed out, and I've fixed my custom dependency to re-raise the exception as it should.

And @alv2017 yes, I can indeed also see the warning that I'm logging (where I should have been re-raising an exception), in my logs.

So, yes, this was a bug in my code, and it's a gotcha that's already covered in the FastAPI docs. But, nevertheless, I feel that there's room for improvement here on the FastAPI side: FastAPI can and should re-raise RuntimeError: generator didn't yield from an async dependency, to make it a more informative exception; and I feel that the FastAPI docs should specifically include an example of a badly-behaved async dependency (currently they only include an example of a synchronous dependency). As such, I've submitted #13264 to implement those improvements.