Suppose a movie-search app calls an API. If that API is slow or unavailable, a unit test of the app’s own search logic should not fail for reasons outside the code being tested. Mocking replaces a real dependency with a fake, controlled version during a test. The test can then choose the dependency’s response without making a network request. section 11.3.1
Choose the mock at the boundary of the component under test:
- For a backend test, replace `requests.get` and provide a fake API response. Check both the returned movie data and the arguments sent to the request function. section 11.3.2
- For a frontend test, replace `Backend` with a class that has the methods the frontend expects but returns fixed movie results and a dummy poster URL. section 11.4.1
A controlled movie-search response
Decorate a backend test with `@patch('requests.get')`. Create a `Mock` response, set `mock_response.json.return_value` to `{'results': MOCK_MOVIES}`, and make the patched request return that response. Call `backend.search_movies('test query')`, then assert that the request used the expected URL and parameters and that the returned movies match the test data. Change the fake results to an empty list to test the no-results case without relying on the live API. section 11.3.2
Listing 11.5 Backend unit tests
import pytest
from unittest.mock import patch, Mock
from backend import Backend, SEARCH_URL
from mock_data import MOCK_MOVIES, MOCK_RESULTS
@pytest.fixture
def backend():
api_key = "test_api_key"
return Backend(api_key)
@patch('requests.get')
def test_search_movies_success(mock_get, backend):
mock_response = Mock()
mock_response.json.return_value = {'results': MOCK_MOVIES}
mock_get.return_value = mock_response
results = backend.search_movies("test query")
mock_get.assert_called_once_with(
SEARCH_URL,
params={"api_key": backend.api_key, "query": "test query"}
)
assert len(results) == len(MOCK_MOVIES)
for _id, movie in MOCK_RESULTS.items():
assert _id in results
assert results[_id]["title"] == movie["title"]
Shows the fixture, patched request, controlled JSON response, and assertions together in one backend test. section 11.3.2
Mocking keeps a unit test focused; it does not show that the real components work together. Use integration tests to check components together and end-to-end tests to check the full flow without mocking. section 11.3.1 section 11.8
