RuntimeError for Depends on Python 3.8 stating "...yield require Python 3.7 or above" when testing Router in pytest

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Example

Here’s a self-contained, minimal, reproducible, example with my use case:

from fastapi import APIRouter, Depends
from fastapi.testclient import TestClient

def get_db():
    db = 1
    yield db

router = APIRouter()
client = TestClient(router)

@router.get("/")
def foo(ds=Depends(get_db)):
    print(ds)
    return {}

def test_foo():
    client.get("/")
    assert 1 == 1

Description

The above sample fails with:

RuntimeError: FastAPI dependencies with yield require Python 3.7 or above, or the backports for Python 3.6, installed with: pip install async-exit-stack async-generator

However, replacing router = FastAPI() with router = FastAPI() succeeds.

I ran into this problem while trying to do some isolated unit testing on the API routes, and I assumed it would have been okay based on the documentation stating:

You can think of APIRouter as a “mini FastAPI” class. All the same options are supported. All the same parameters, responses, dependencies, tags, etc.

The workaround is simple, and I can first create an app class in my testing, but I’m more curious about why this doesn’t work. Unless I’m doing something incorrectly, maybe this is worth a documentation note, or a clearer exception message?

Environment

• OS: macOS 10.15.7
• FastAPI Version: 0.61.1
• Python version: 3.8.6

[bluesliverx]

This should be documented as a gotcha, etc, just in case someone else runs into exactly the same thing. Maybe in the section about routers or maybe in the section about testing, not sure. I’m happy to create a PR if someone makes a decision on where it should go.

[github-actions[bot]]

Assuming the original need was handled, this will be automatically closed now. But feel free to add more comments or create new issues or PRs.