How to mock google bigquery client for unittests in pytests

Question

Below I am using a python client library to connect to bigquery and a service account to connect to the bigquery (see this link to get more info about the library).

from google.cloud.bigquery import Client, LoadJobConfig, LoadJob
from google.oauth2 import service_account
     
def fetch_last_modified_date_from_bq():
    service_account_json_string = "json to connect"
    service_account_json = json.loads(service_account_json_string)
    credentials = service_account.Credentials.from_service_account_info(service_account_json)
    client = Client(credentials=credentials)        
    # Run a SQL query on the table
    sql = """
                  SELECT last_modified_date FROM `project.dataset.table` 
                  order by last_modified_date desc LIMIT 1
          """
    query_job = client.query(sql)
    # Print the results
    for row in query_job:
        return row.result //this is string

How to mock the client?

I tried to mock in this way but as client needs "credentials" to be passed and it was not successful.
Is the code below the right way?

@mock.patch('google.cloud.bigquery.Client')        
@mock.patch('google.oauth2.service_account.Credentials.from_service_account_info')
@mock.patch('google.auth.credentials.Credentials')
def test_fetch_last_modified_date_from_bq(self, mock_credentials ,mock_service_account, mock_client):
    #arrange
    mock_service_account.return_value = mock_credentials
    row={}
    row['last_modified_date'] = ''
    rows = [row]
    mock_client.return_value.query.return_value = rows
    #act
    fetch_last_modified_date_from_bq()

Inserted

What is the method biquery_store.fetch_last_modified_date_from_bq() which you call by the test method? – Lisk

Does this link help you? – Archine

@frankfalse check now I updated the question – Inserted

@kiranmathew Nopes, In that link mock_client is no way related to service account and credentials. In my case I need to mock both service_account_info and credentials. – Inserted

@DeepakKothari In the function/method fetch_last_modified_date_from_bq() you assign somethin to credentials two times? Why? – Lisk

@frankfalse corrected it. That was typo mistake – Inserted

@DeepakKothari The function fetch_last_modified_date_from_bq() and the test test_fetch_last_modified_date_from_bq() are in the same file? – Lisk