[BUG] Async class method dependency raises a ValueError

Describe the bug If you use an async class method as a dependency, a ValueError is thrown. It doesn’t happen for a non-async method.

Complete error: ValueError: [KeyError(<class 'coroutine'>), TypeError("'coroutine' object is not iterable"), TypeError('vars() argument must have __dict__ attribute')] (at fastapi/encoders.py:106)

To Reproduce

from fastapi import Depends, FastAPI
from starlette.requests import Request

class DependencyClass:
    async def async_dep(self, request: Request):
        return True

    def sync_dep(self, request: Request):
        return True

app = FastAPI()
dependency = DependencyClass()

# Error
@app.get('/async-dep')
def async_dep(r=Depends(dependency.async_dep)):
    return r

# Everything is fine
@app.get('/sync-dep')
def sync_dep(r=Depends(dependency.sync_dep)):
    return r

Expected behavior The async class method dependency should be called and its return value injected.

Environment:

• OS: macOS
• FastAPI Version: 0.42.0
• Python version: 3.7.2

Additional context I believe the issue comes from here:

https://github.com/tiangolo/fastapi/blob/65536cbf63318d111bf608960378d651b6c1596a/fastapi/dependencies/utils.py#L353-L359

Indeed, inspect.isfunction(call) will return False in case of a class method. Hence, it is sent to run_in_threadpool, which never awaits the coroutine, and we end up trying to serialize it instead of its result (hence the ValueError).

Changing the check by:

if inspect.isfunction(call) or inspect.ismethod(call):

solves the issue. I can make a PR with the fix and unit tests if it helps.

[tiangolo]

Thanks for the help here everyone! 👏 🙇

Thanks for reporting back and closing the issue @frankie567 👍 And thanks for the PR/fix! 🎉

[dmontagu]

This makes sense to me; @frankie567 I think it’s worth a PR.