Unable to mock method using pytest-mock - pytest

Question

I have a class 'MyClass' with code as below.
class MyClass:
def __init__(update_type_id='1')
self.update_type_id = update_type_id
self._cursor = <database_connection).cursor()
def update_start_dt(self):
self._update_job_ctrl_start_dt()
def _update_job_ctrl_start_dt(self):
update_sql, v_1 = self._get_update_sql(self._update_type_id)
logger.debug(f'Update sql: {update_sql} and v_1: {v_1}')
def _get_update_sql(self, update_type_id: int) -> Tuple:
sql = f"SELECT start_sql, end_sql FROM <database.table> where update_type_key = {self._update_type_id}"
self._run_sql(sql)
record = self._cursor.fetchone()
if record:
return record
else:
logger.error(f'Record Not Found. Update type key ({update_type_id}) not found in the table in the database')
raise Exception
def _run_sql(self, sql_statement: str):
try:
self._cursor.execute(sql_statement)
except (Exception, Error) as e:
logger.error(f'Error {e} encountered when reading from table')
raise e
I am trying to write a test function using pytest-mock which will test the update_start_dt method. The method internally invokes a series of private methods and I am having difficulty in mocking the code which runs through all the private methods. Can anyone help me to understand in what all ways we can mock?
I tried to refer multiple online websites but couldn't get a complete picture.
class TestMyClass:
def test_update_start_dt(mocker,mock_get_connection,mock_run_sql):
mock_manager = mocker.Mock()
mock_get_update_sql = mock_manager.patch('MyClass._get_update_sql')
mock_get_update_sql.return_value = ('123','234')
myclass = MyClass(update_type_id='1')
myclass.update_start_dt()
I am getting error as below for above test code
update_sql, v_1 = self._get_update_sql(self._update_type_id)
ValueError: not enough values to unpack (expected 2, got 0)

Answer 1

The issue here is that you are patching on a Mock object that you are creating, for the purposes of your test you do not need to explicitly create a Mock object. Shown below is how you would test it instead, where we patch directly on the class.
class MyClass:
def __init__(self, update_type_id='1'):
self._update_type_id = update_type_id
self._cursor = None
def update_start_dt(self):
self._update_job_ctrl_start_dt()
def _update_job_ctrl_start_dt(self):
update_sql, v_1 = self._get_update_sql(self._update_type_id)
logger.debug(f'Update sql: {update_sql} and v_1: {v_1}')
def _get_update_sql(self, update_type_id: int):
sql = f"SELECT start_sql, end_sql FROM <database.table> where update_type_key = {self._update_type_id}"
self._run_sql(sql)
record = self._cursor.fetchone()
if record:
return record
else:
logger.error(f'Record Not Found. Update type key ({update_type_id}) not found in the table in the database')
raise Exception
def _run_sql(self, sql_statement: str):
try:
self._cursor.execute(sql_statement)
except (Exception, Error) as e:
logger.error(f'Error {e} encountered when reading from table')
raise e
def test_update_start_dt(mocker):
mock_get_update_sql = mocker.patch.object(MyClass, "_get_update_sql")
mock_get_update_sql.return_value = ("123", "234")
myclass = MyClass(update_type_id='1')
myclass.update_start_dt()
=========================================== test session starts ============================================
platform darwin -- Python 3.8.9, pytest-7.0.1, pluggy-1.0.0
rootdir: ***
plugins: mock-3.7.0
collected 1 item
test_script.py . [100%]
======================================= 1 passed, 1 warning in 0.01s =======================================
Your code would work if you called the Mock object you created instead of the class. That is shown below.
def test_update_start_dt(mocker):
mock_manager = mocker.Mock()
mock_get_update_sql = mock_manager.patch('MyClass._get_update_sql')
mock_get_update_sql.return_value = ('123','234')
# Notice how we use `mock_manager` instead of MyClass
# tests will now pass
myclass = mock_manager(update_type_id='1')
myclass.update_start_dt()
Hopefully you see what the issue is now.