I have a fastapi router file that has a function that calls a utility function in another file that interacts with GCS. I want to mock out the entire gcsutil.create_bucket in an unit test of institutions.py:

institutions.py

from google.cloud import storage
from ..gcsutil import generate_upload_signed_url, create_bucket, create_folders
# .... other imports

router = APIRouter(
    tags=["institutions"],
)

@router.post("/institutions/", response_model=Institution)
def create_institution(
    req: InstitutionCreationRequest,
    current_user: Annotated[BaseUser, Depends()],
    sql_session: Annotated[Session, Depends(get_session)],
) -> Any:
    bucket_name = "FOO"
    try:
        # WANT TO MOCK OUT THE BELOW create_bucket
        create_bucket(bucket_name)
    except ValueError as e:
        raise HTTPException(...SOME ERROR...)
    return {}

gcsutil.py

from google.cloud import storage, storage_control_v2
from google.cloud.storage import Client
from typing import Any

def create_bucket(bucket_name: str) -> Any:
    client = storage.Client()
    bucket = client.bucket(bucket_name)
    if bucket.exists():
        raise ValueError(bucket_name + " already exists. Creation failed.")
    bucket.storage_class = "STANDARD"
    new_bucket = client.create_bucket(bucket, location="us")
    return new_bucket

institutions_test.py

from fastapi.testclient import TestClient
import pytest
import unittest
from unittest import mock
from unittest.mock import Mock

from . import institutions
from ..main import app

@pytest.fixture(name="session")
def session_fixture():
    # sets up databases

@pytest.fixture(name="client")
def client_fixture(session: sqlalchemy.orm.Session):
    def get_session_override():
        return session

    app.include_router(institutions.router)
    app.dependency_overrides[get_session] = get_session_override

    client = TestClient(app)
    yield client
    app.dependency_overrides.clear()

mock.patch(institutions.__name__ + '.create_bucket', Mock(return_value=None))
def test_create_inst(mock_storage, client):
    mock_gcs_client = mock_storage.Client.return_value
    mock_bucket = Mock()
    mock_bucket.blob.return_value.download_as_string.return_value = "foobar".encode('utf-8')
    mock_gcs_client.bucket.return_value = mock_bucket

    response = client.post("/institutions/12345")
    assert response.status_code == 200
    

Based on @Patch decorator is not compatible with pytest fixture and How to mock using patch relative paths?

However I’m getting  this error:

fixture 'mock_create_bucket' not found

>       available fixtures: _session_faker, anyio_backend, anyio_backend_name, anyio_backend_options, cache, capfd, capfdbinary, caplog, capsys, capsysbinary, client, doctest_namespace, faker, monkeypatch, pytestconfig, record_property, record_testsuite_property, record_xml_attribute, recwarn, session, tmp_path, tmp_path_factory, tmpdir, tmpdir_factory

>       use 'pytest --fixtures [testpath]' for help on them.