How to mock pip package in pytest

This is a note about what I did to mock pip package in pytest last month

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Background

I’m working on firmware development with Python and writing unit tests using pytest. Then, I faced cases where the firmware depended on the hardware interface, such as I/O pins, Bus, and so on. When I mocked the codes made by myself, mocking the interfaces was straightforward. However, it wasn't that straightforward when the interfaces were used in pip packages like below. I extracted the related code lines.

# classes/led.py
import some.GPIO.library as GPIO

class LED:
    def __init__():
        GPIO()

# classes/manager.py
from classes.led import LED

class Manager:
     def __init__():
          LED()

# test_led.py
@pytest.fixture
def import_led_with_mocked_gpio():
    mock_gpio_a = MagicMock()
    sys.modules["some.GPIO.library"] = mock_gpio

    from classes.led import LEDController

    return (mock_gpio_a, LEDController)

# test_manager.py
@pytest.fixture
def import_manager_with_mocked_gpio():
    mock_gpio_b = MagicMock()
    sys.modules["some.GPIO.library"] = mock_gpio

    from classes.manager import Manager

    return (mock_gpio_b, Manager)

I tried to assert whether the mock_gpio_bis called when the Manageris initialized. But it didn’t work as I expected. So I started to investigate.

What I did

First of all, I checked the import implementation and found the following code line.

cpython/Lib/importlib/_bootstrap.py at d112c600ab3f5e2910345bcc4bfc39439917ed87 · python/cpython

From the code line, Python caches the libraries which has been already imported before in sys.modules . That’s why I couldn’t mock to import some.GPIO.library as I expected. What happened to me was

1. some.GPIO.library is mocked with mock_gpio_a when the test_led.py is executed
2. classes.led is cached with mock_gpio_a in sys.modules
3. sys.modules returns cached classes.led with mock_gpio_a when the test_manager.py is executed

Then, eventually, mock_gpio_b.assert_called_once() didn’t work because the some.GPIO.library was mocked in classes.led.py with mock_gpio_a and cached in sys.modules . So I created the fixture to clear the cache.

@pytest.fixture
def clear_system_module_cache():
    if "classes.leds" in sys.modules:
        del sys.modules["classes.leds"]

    yield

    del sys.modules["classes.leds"]

Then the initial code snippets became below.

# test_led.py
@pytest.fixture
def clear_system_module_cache():
    if "classes.leds" in sys.modules:
        del sys.modules["classes.led"]

    yield

    del sys.modules["classes.led"]

@pytest.fixture
def import_led_with_mocked_gpio(clear_system_module_cache):
    mock_gpio_a = MagicMock()
    sys.modules["some.GPIO.library"] = mock_gpio

    from classes.led import LEDController

    return (mock_gpio_a, LEDController)

# test_manager.py
@pytest.fixture
def import_manager_with_mocked_gpio():
    mock_gpio_b = MagicMock()
    sys.modules["some.GPIO.library"] = mock_gpio

    from classes.manager import Manager

    return (mock_gpio_b, Manager)

After that, the mock_gpio_b worked as I expected.

That’s it!