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Module 08 / Project 02 — Mocking

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Focus

  • unittest.mock.patch to replace external dependencies during tests
  • MagicMock for creating fake objects with any attributes or methods
  • side_effect for simulating errors and varying responses
  • return_value for controlling what a mock returns

Why this project exists

Real applications talk to external services — APIs, databases, file systems. You cannot run tests that depend on a live weather API: the tests would be slow, flaky (what if the API is down?), and non-deterministic (weather changes). Mocking lets you replace the real API call with a fake one that returns exactly what you tell it to. Your tests run in milliseconds, work offline, and test your logic without testing someone else's server.

Run

cd projects/modules/08-testing-advanced/02-mocking
pytest tests/test_weather.py -v

Expected output

tests/test_weather.py::test_get_temperature_success PASSED
tests/test_weather.py::test_get_temperature_city_not_found PASSED
tests/test_weather.py::test_get_temperature_api_error PASSED
tests/test_weather.py::test_get_temperature_timeout PASSED
tests/test_weather.py::test_get_forecast_success PASSED
tests/test_weather.py::test_get_forecast_returns_correct_days PASSED
tests/test_weather.py::test_get_temperature_different_cities PASSED

All tests should pass without making any real HTTP requests.

Alter it

  1. Add a new method get_humidity(city) to WeatherService and write a mocked test for it.
  2. Change one of the mocked responses to return a different temperature and verify the test still checks the right thing.
  3. Add a test that verifies requests.get was called with the correct URL (use mock.assert_called_once_with).

Break it

  1. Remove the @patch decorator from one test and run it. What happens when the test tries to call the real API?
  2. Change the patch target to the wrong module path (e.g., @patch("requests.get") instead of @patch("project.requests.get")). Why does this matter?
  3. Make a mock return a response without a .json() method and see what error your code raises.

Fix it

  1. Put the @patch decorator back and verify the test passes again.
  2. Fix the patch target so it points to the right location.
  3. Add proper error handling in WeatherService for responses that do not have the expected JSON structure.

Explain it

  1. Why do we patch project.requests.get instead of just requests.get?
  2. What is the difference between return_value and side_effect?
  3. What happens if you forget to patch an external dependency in a test?
  4. How does MagicMock know what attributes and methods to have?

Mastery check

You can move on when you can:

  • Write a @patch decorator from memory and explain the target path.
  • Use side_effect to simulate both exceptions and varying return values.
  • Explain why mocking makes tests faster and more reliable.
  • Verify that a mock was called with specific arguments.

Next

Project 03 — Fixtures Advanced