[QUESTION] How to get dependency_overrides to work

I’m not sure whether I’m just doing something wrong or if this is a bug. I’m on version 0.42.0. I’m trying to use dependency_overrides to override a function for testing, but I can’t get it to work. Here’s a basic example.

routers/some_router.py

from typing import List

from fastapi import APIRouter

router = APIRouter()


def some_func():
    return ['foo']


@router.get('/some-endpoint', response_model=List[str])
def some_endpoint() -> List[str]:
    return some_func()

test_some_router.py

from fastapi import FastAPI
from starlette.testclient import TestClient

from routers import some_router

app = FastAPI()
app.include_router(some_router.router)

client = TestClient(app)


def some_func_new():
    return ['foo', 'bar', 'baz', 'qux']


app.dependency_overrides[some_router.some_func] = some_func_new


def test_some_endpoint():
    response = client.get('/some-endpoint')
    assert response.status_code == 200
    assert response.json() == ['foo', 'bar', 'baz', 'qux']

And the result:

===================== FAILURES =====================
________________ test_some_endpoint ________________

    def test_some_endpoint():
        response = client.get('/some-endpoint')
        assert response.status_code == 200
>       assert response.json() == ['foo', 'bar', 'baz', 'qux']
E       AssertionError: assert ['foo'] == ['foo', 'bar', 'baz', 'qux']

I’m not sure if I should be importing things or setting them up in a different way. I’ve also tried from main import app, some_router as well as directly importing some_func. If I’m doing something wrong, I think the documentation could be made clearer (e.g. a two file example instead of having both the API and tests in the same file).

[larissaleite]

In the end, for the context of my project, we decided to go with not touching anything else but the client fixture, and not use dependency_overrides, at least in the mean time:

@pytest.fixture
def client(mocker):
    mocker.patch("app.routers.my_router.permission_required", return_value=lambda: None)
    app = FastAPI()
    app.include_router(my_router.router())
    client = TestClient(app)
    yield client

This does the trick for us, hopefully it can also help others!

[larissaleite]

Thanks @Kludex for going deeper into this, I appreciate it! It’d be indeed good to understand whether this is a limitation of FastAPI or if there’s a possible way around it. In the mean time, I will check whether I can refactor my function to not take any parameters