Using app.dependency_overrides with parametrized depenencies

[shohamy7]

Hi!
I'm trying to test my FastAPI application and got into some trouble with app.dependency_overrides.
I'm using parametrized dependencies and when I'm trying to add the dependency to the dependency_overrides dict and run my tests, the tests runs with the original dependency instead of the mock dependency I created.
When I use regular function, it works fine. The problem occurs only with class dependencies.
Here what I tried to do:

class NameDependency:
    def __init__(self, name: str):
        self._name = name
    def __call__(self) -> str:
        return self._name

app = FastAPI()

@app.get('/')
async def hello(name: str = Depends(NameDependency('foo'))):
    return {'message': f'hello {name}'}

client = TestClient(app)
app.dependency_overrides[NameDependency] = Mock(return_value='bar')

def test_hello():
    res = client.get('/')
    assert res.json()['message'] == 'hello bar'

When I run this test, instead of passing, I get AssertionError hello foo == hello bar.
I also tried the following alternatives:

# 1st alternative
app.dependency_overrides[NameDependency('foo')] = Mock(return_value='bar')

# 2nd alternative
app.dependency_overrides[NameDependency] = Mock(return_value=Mock(return_value='bar'))

I've tried more but I could not find the correct way to do that.
I think I'm missing something and I'll glad if someone can help me. I didn't find an example to this online and I didn't see this in the docs either.

Thanks for anyone that will help!

[Mause]

It's because your NameDependency class isn't hashable (so the lookups in the dependency_overrides dict is failing).
It's easy to fix with dataclasses:

from fastapi import FastAPI, Depends
from fastapi.testclient import TestClient
from dataclasses import dataclass


@dataclass(frozen=True, eq=True)
class NameDependency:
    name: str

    def __call__(self) -> str:
        return self.name


app = FastAPI()


@app.get("/")
async def hello(name: str = Depends(NameDependency("foo"))):
    return {"message": f"hello {name}"}


client = TestClient(app)
app.dependency_overrides[NameDependency("foo")] = lambda: "bar"


def test_hello():
    res = client.get("/")
    assert res.json()["message"] == "hello bar"

[shohamy7]

That's works, Thank you!
I wish this would have been written in the docs, it would save me a lot of time :)

[delijati]

It's because your NameDependency class isn't hashable (so the lookups in the dependency_overrides dict is failing).
It's easy to fix with dataclasses:

from fastapi import FastAPI, Depends
from fastapi.testclient import TestClient
from dataclasses import dataclass


@dataclass(frozen=True, eq=True)
class NameDependency:
    name: str

    def __call__(self) -> str:
        return self.name


app = FastAPI()


@app.get("/")
async def hello(name: str = Depends(NameDependency("foo"))):
    return {"message": f"hello {name}"}


client = TestClient(app)
app.dependency_overrides[NameDependency("foo")] = lambda: "bar"


def test_hello():
    res = client.get("/")
    assert res.json()["message"] == "hello bar"

Took me some time to find this ... Should definitely go to the docs or faq. Thanks a lot @Mause
To get this working on any class you have to overwrite __eg__ and __hash__:

class JWTBearer(HTTPBearer):
    def __init__(self, auto_error: bool = True):
        super().__init__(auto_error=auto_error)

    def __hash__(self):
        # FIXME find something uniq and repeatable                                     
        return 1234   
   
    def __eq__(self, other):                       
        """Overrides the default implementation"""
        if isinstance(other, JWTBearer):
            return self.auto_error == other.auto_error                  
        return False

[alijewers]

Would be really great if this could make it into the documentation, our team has ended up back here several times now after encountering this problem independently