Question

I have this small MCVE FastAPI application which works fine as expected:

# run.py
import uvicorn
from fastapi import FastAPI, Depends

app = FastAPI()


###########

def dep_with_arg(inp):
    def sub_dep():
        return inp
    return sub_dep


@app.get("/a")
def a(v: str = Depends(dep_with_arg("a"))):
    return v


###########


def dep_without_arg():
    return "b"


@app.get("/b")
def b(v: str = Depends(dep_without_arg)):
    return v


###########


def main():
    uvicorn.run(
        "run:app",
        host="0.0.0.0",
        reload=True,
        port=8000,
        workers=1
    )


if __name__ == "__main__":
    main()

notice the difference between the dependencies of the /a and /b endpoints. in the /a endpoint, I have created a dependency that accepts an argument. both endpoints are working as expected when I call them.

I now try to test the endpoints while overriding the dependencies as below:

from run import app, dep_without_arg, dep_with_arg
from starlette.testclient import TestClient


def test_b():
    def dep_override_for_dep_without_arg():
        return "bbb"

    test_client = TestClient(app=app)
    test_client.app.dependency_overrides[dep_without_arg] = dep_override_for_dep_without_arg
    resp = test_client.get("/b")
    resp_json = resp.json()
    assert resp_json == "bbb"


###########


def test_a_method_1():
    def dep_override_for_dep_with_arg(inp):
        def sub_dep():
            return "aaa"

        return sub_dep

    test_client = TestClient(app=app)
    test_client.app.dependency_overrides[dep_with_arg] = dep_override_for_dep_with_arg
    resp = test_client.get(
        "/a",
    )
    resp_json = resp.json()
    assert resp_json == "aaa"


def test_a_method_2():
    def dep_override_for_dep_with_arg(inp):
        return "aaa"

    test_client = TestClient(app=app)
    test_client.app.dependency_overrides[dep_with_arg] = dep_override_for_dep_with_arg
    resp = test_client.get(
        "/a",
    )
    resp_json = resp.json()
    assert resp_json == "aaa"


def test_a_method_3():
    def dep_override_for_dep_with_arg(inp):
        return "aaa"

    test_client = TestClient(app=app)
    test_client.app.dependency_overrides[dep_with_arg] = dep_override_for_dep_with_arg("aaa")
    resp = test_client.get(
        "/a",
    )
    resp_json = resp.json()
    assert resp_json == "aaa"


def test_a_method_4():
    def dep_override_for_dep_with_arg():
        return "aaa"

    test_client = TestClient(app=app)
    test_client.app.dependency_overrides[dep_with_arg] = dep_override_for_dep_with_arg
    resp = test_client.get(
        "/a",
    )
    resp_json = resp.json()
    assert resp_json == "aaa"

test_b passes as expected but all the other tests fail:

FAILED test.py::test_a_method_1 - AssertionError: assert  a  ==  aaa 
FAILED test.py::test_a_method_2 - AssertionError: assert  a  ==  aaa 
FAILED test.py::test_a_method_3 - AssertionError: assert  a  ==  aaa 
FAILED test.py::test_a_method_4 - AssertionError: assert  a  ==  aaa

How should I override dep_with_arg in the example above?

Answer 1

The error you are getting is because the dep_with_arg dependency expects an argument, but the dep_override_for_dep_with_arg function you are passing to app.dependency_overrides does not take any arguments.

To fix this, you need to update the dep_override_for_dep_with_arg function to take an argument and return the value of that argument. For example:

def dep_override_for_dep_with_arg(inp):
    def sub_dep():
        return inp

    return sub_dep

This will allow you to override the dep_with_arg dependency with the value "aaa". For example:

test_client = TestClient(app=app)
test_client.app.dependency_overrides[dep_with_arg] = dep_override_for_dep_with_arg("aaa")
resp = test_client.get(
    "/a",
)
resp_json = resp.json()
assert resp_json == "aaa"

This will now pass the test.

Here is the full code with the fix:

import uvicorn
from fastapi import FastAPI, Depends

app = FastAPI()


def dep_with_arg(inp):
    def sub_dep():
        return inp
    return sub_dep


@app.get("/a")
def a(v: str = Depends(dep_with_arg("a"))):
    return v


def dep_without_arg():
    return "b"


@app.get("/b")
def b(v: str = Depends(dep_without_arg)):
    return v


def dep_override_for_dep_with_arg(inp):
    def sub_dep():
        return inp

    return sub_dep


def test_b():
    def dep_override_for_dep_without_arg():
        return "bbb"

    test_client = TestClient(app=app)
    test_client.app.dependency_overrides[dep_without_arg] = dep_override_for_dep_without_arg
    resp = test_client.get("/b")
    resp_json = resp.json()
    assert resp_json == "bbb"


def test_a():
    test_client = TestClient(app=app)
    test_client.app.dependency_overrides[dep_with_arg] = dep_override_for_dep_with_arg("aaa")
    resp = test_client.get("/a")
    resp_json = resp.json()
    assert resp_json == "aaa"


if __name__ == "__main__":
    uvicorn.run(
        "run:app",
        host="0.0.0.0",
        reload=True,
        port=8000,
        workers=1
    )

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