[Python] Mocking of a function within an api call doesn't work in FastAPI

I'm trying to mock a function which is being called within an API call in a FastAPI project. But the mock doesn't work. I've prepared a sample API to demonstrate the issue.

conftest.py

@pytest.fixture(scope="function")
def get_client():
client = TestClient(app)
yield client


utils.py

def sample_function() -> str:
return "Hello, World!"


api.py

from src.utils import sample_function

@router.get(
"/test-api",
status_code=status.HTTP_200_OK,
)
def sample_api() -> JSONResponse:
return JSONResponse(
status_code=status.HTTP_200_OK,
content=sample_function(),
)


tests.py

from src.api import sample_api

@patch("src.path.to.api.sample_function")
def test_sample_api(
mock_sample_function,
get_client,
):

mock_sample_function.return_value = "Bye World!"

response = get_client.get(
"/test-api",
)
assert json.loads(response.body) == "Bye World!"

@patch("src.path.to.api.sample_function")
def test_sample_api_function(
mock_sample_function,
):

mock_sample_function.return_value = "Bye World!"

response = sample_api()
assert json.loads(response.body) == "Bye World!"


In this sample if I try test_sample_api_function mocking works well, but if I try the test which actually test the api call test_sample_api, it can't mock the sample_function method.

Why is this happening and what am I missing here?

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