[Python] how to unit test flask method using google cloud storage library

Im trying to create unit tests for this method but im getting the error saying credentials not found

from io import BytesIO

from google.auth.exceptions import DefaultCredentialsError
from google.cloud import storage
from fastapi import HTTPException

storage_client = storage.Client()
bucket_name = "stupid-bucket-name-1234567890"

async def upload_to_gcs(file, file_name: str) -> str:

try:
bucket = storage_client.bucket(bucket_name)
blob = bucket.blob(file_name)
binary_data = await file.read()
blob.upload_from_file(BytesIO(binary_data), content_type=file.content_type)
file_url = blob.public_url
return file_url
except DefaultCredentialsError as exe:
raise HTTPException(status_code=500, detail=str(exe))
except Exception as e:
raise HTTPException(status_code=500, detail=str(e))


now here is my unit test code

import sys
import pytest
from unittest.mock import patch, MagicMock, AsyncMock
from fastapi import HTTPException
from google.auth.exceptions import DefaultCredentialsError

# Patch BEFORE importing cloud_service
with patch("package.svc.cloud.storage.Client") as mock_client:
mock_storage_client = mock_client.return_value
mock_bucket = MagicMock()
mock_blob = MagicMock()
mock_storage_client.bucket.return_value = mock_bucket
mock_bucket.blob.return_value = mock_blob
mock_blob.public_url = "https://fake-url.com/file.txt"
import importlib
cloud_service = importlib.import_module("package.svc.cloud")

@pytest.mark.asyncio
async def test_upload_file_to_gcs_success():
mock_file = AsyncMock()
mock_file.read.return_value = b"test data"
mock_file.content_type = "text/plain"
url = await cloud_service.upload_file_to_gcs(mock_file, "file.txt")
assert url == "https://fake-url.com/file.txt"
cloud_service.storage_client.bucket.return_value.blob.return_value.upload_from_file.assert_called_once()


but when I run this, it says: Failed to load service account credentials from ... No key could be found

I don't understand why it is looking for the actual keys even though I have already mocked the lines in the code that would return the Client object.

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