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

[lieryan]

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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

[Kludex]

Yeah. I think FastAPI would need to unwrap the partial, like it's being done for everything else in Starlette: https://github.com/encode/starlette/blob/master/starlette/_utils.py#L14-L15

[Kludex]

There was a PR that was closed some time ago: #3343

[lieryan]

I've written an updated PR that would implement support for this and other uses of partials on the various forms of dependables: #9753. Unlike #3343, it also ensures that partial will also work reliably in all sorts of async/callable dependables, in addition to async def.

[realitycheck]

Any chances of #9753 being merged and released soon?

[Kludex]

I've created this: #10720