Mock/Patch specific object method inside a class method

Question

I know the title is quite confusing, but for the sake of clarity, I created a sample of what I have to transpose:

Let's say I have this code in class_file.py

class CarDealership:
    def allow_car_out_of_the_dealership(self, car):
        logger.info(f"Driving out of the dealership")
        
        drivers_license = car.driver.wallet.get_drivers_license()

        if drivers_license:
            try:
                allow_car_removal(car, drivers_license)
            except:
                deny_car_removal(car, drivers_license)

I'm trying to test a situation where the method deny_car_removal is called. get_valid_license() is a method specific to driver.wallet and is not otherwise imported or referenced in class_file.py in any way, shape or form.

The problem is that the code goes a completely different way if get_valid_license does not return anything valid in the real situation. I'm trying to patch the drivers_license so I can eventually bring it to the point I need.

I'm guessing test_class_file.py should look something like this:

class CarDealershipTests(BaseTestCase):
    @patch('class_file.CarDealership.deny_car_removal')
    def test_deny_car_removal(self):
        # something here
        self.assertTrue(mock_deny_car_removal.called)

A few things I have tried from looking at other StackOverflow answers but didn't work:

• In the test setup, mock a full driver instance inside the mocked car, like so:

    def setUp(self) -> None:
        self._create_car
    
    def _create_car(self):
        car = Car()
        car.driver = MagicMock()

• Tried using @patch.object(class_file.CarDealership.allow_car_out_of_the_dealership, "car.driver.wallet.get_valid_license")
• Tried using @patch.object(class_file.CarDealership.allow_car_out_of_the_dealership, "drivers_license")
• Tried using @patch("class_file.CarDealership.allow_car_out_of_the_dealership.car.driver.wallet.get_valid_license")
• Tried using @patch("class_file.CarDealership.allow_car_out_of_the_dealership.drivers_license")

Answer 1

Suppose you have a file with the following contents as you have shown:

import logging

logger = logging.getLogger(__name__)

class CarRemovalFailure(BaseException):
    pass


class CarDealership:
    def allow_car_out_of_the_dealership(self, car):
        logger.info("Driving out of the dealership")
        
        drivers_license = car.driver.wallet.get_drivers_license()

        if drivers_license:
            try:
                allow_car_removal(car, drivers_license)
            except CarRemovalFailure:
                deny_car_removal(car, drivers_license)

In order to have deny_car_removal be called, we will need allow_car_removal to throw an Exception. This is easily done by using the side_effect attribute of the Mock object.

I didn't incorporate any setup/tear-down up in this test but instead am illustrating how side_effect works and how it ends up having deny_car_removal be called.

from unittest.mock import MagicMock, patch
from class_file import CarDealership, CarRemovalFailure


@patch("class_file.allow_car_removal")
@patch("class_file.deny_car_removal")
def test_deny_car(mock_deny, mock_allow):
    mock_allow.side_effect = CarRemovalFailure("Failed!")
    car = MagicMock()
    cd = CarDealership()

    cd.allow_car_out_of_the_dealership(car)

    car.driver.wallet.get_drivers_license.assert_called_once()
    mock_deny.assert_called_once()

When run it outputs the following:

============================= test session starts ==============================
platform darwin -- Python 3.9.1, pytest-6.2.2, py-1.10.0, pluggy-0.13.1
rootdir: ****
collected 1 item                                                               

tests/test_car.py .                                                      [100%]

============================== 1 passed in 0.05s ===============================

In your example you might not want to patch deny_car_removal as you are trying to test the logic, but since I don't know what that might be I patched it on my end. The point of that was to illustrate that in fact the function does get called.