functools.partial() does not work on an async dependable

[Kludex]

Discussed in #9744

^{Originally posted by lieryan June 26, 2023}

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Example Code

from functools import partial

from fastapi import FastAPI, Depends


app = FastAPI()

def the_answer():
    return 42


async def dep1(
    n: int = Depends(the_answer),
):
    return n + 1

@app.get("/")
def main(
    d1=Depends(partial(dep1, n=10)),
):
    print(d1)   # prints "<coroutine object dep1 at 0x7facc996fb50>"
    return {}


if __name__ == '__main__':
    import uvicorn
    uvicorn.run(app, port=1122)

Description

I would've expected d1 above to be the return value of the dependable, but it seems like fastapi didn't identify the partial as an coroutine function and instead passed the unawaited coroutine into the view function.

inspect.iscoroutinefunction() and inspect.signature() were able to deconstruct the partial object to correctly identify whether a partial contains a coroutine or a regular function, so it seems like it should've been possible for FastAPI to use handle partial correctly:

In [5]: def regular(a, b): pass

In [6]: async def coro(a, b): pass

In [7]: inspect.iscoroutinefunction(partial(regular, 1))
Out[7]: False

In [8]: inspect.iscoroutinefunction(partial(coro, 1))
Out[8]: True

In [20]: inspect.signature(partial(coro, 1))
Out[20]: <Signature (b)>

Operating System

Linux

Operating System Details

No response

FastAPI Version

0.98.0

Python Version

Python 3.10.6

Additional Context

No response

[dayeondev]

I understand that using an asynchronous function without await in a synchronous function will naturally result in a coroutine object. Could you please clarify the intention behind your question? It seems evident that this behavior is expected in Python's asynchronous programming model.

[knight2995]

I understand that using an asynchronous function without await in a synchronous function will naturally result in a coroutine object. Could you please clarify the intention behind your question? It seems evident that this behavior is expected in Python's asynchronous programming model.

In my opinion,in FastAPI, dependency functions are automatically executed with await added.
This is based on the documentation

[atapin]

@Kludex it works as expected with yield:

async def dep1(
    n: int = Depends(the_answer),
):
    yield n + 1

Output 1:

@app.get("/")
def main(
    d1=Depends(partial(dep1)),
):
    print(d1)  # prints 43
    return {}

Output 2:

@app.get("/")
def main(
    d1=Depends(partial(dep1, n=10)),
):
    print(d1)  # prints 11
    return {}

[lieryan]

There is a pull request that has the test cases and the fix for this issue: #9753.

@atapin yeah, async generator dependency works, but it doesn't work for async regular function dependency.