Mocking

Mocking is a testing technique in which a real dependency, such as a database, an HTTP service or the system clock, is replaced by a controlled stand-in. The mock returns scripted responses and can record how it was called, so a test can check behavior without relying on the real thing.

Why it matters

Mocks make unit tests fast, deterministic and independent of the network. They let you simulate rare conditions that are hard to trigger for real, such as a payment gateway timing out or a disk being full. The risk is over-mocking: if a mock does not behave like the real system, tests pass while production fails, and tests tied to exact calls break whenever the implementation changes. Related terms include stubs (canned answers), fakes (simple working versions) and spies (record calls).

Example

from unittest.mock import Mock

http = Mock()
http.get.return_value.status_code = 503

assert check_site("https://example.com", http) == "down"
http.get.assert_called_once_with("https://example.com")

No network request is made, yet the failure path is verified.

Common mistakes

• Mocking the code under test instead of its dependencies.
• Asserting on every internal call, making tests brittle.
• Never running tests against the real dependency in integration tests.

Related

• Unit test
• Integration test
• Dependency injection
• Test-driven development

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