How do you verify and debug mocks/stubs in unit testing?

1 What are mocks and stubs?

Mocks and stubs are two types of test doubles, which are objects that replace the real dependencies of the unit under test. A stub is a simple object that provides predefined responses to the calls it receives, without any logic or verification. A mock is a more complex object that can also record the calls it receives, check the arguments and order of the calls, and verify that the expected interactions occurred. You can use mocks and stubs to test the behavior of the unit under test, without relying on the actual implementation or state of the dependencies.

2 How to create mocks and stubs?

There are different ways to create mocks and stubs, depending on the language and framework you use. In Python, you can use the unittest.mock module, which provides a Mock class that can act as both a mock and a stub. You can create a mock or stub object by instantiating the Mock class, and optionally passing some arguments to configure its behavior. For example, you can specify the return value, the side effect, or the attributes of the mock or stub object. You can also use the patch function or decorator to replace the real dependency with a mock or stub object in a test case or a test suite.

3 How to verify mocks and stubs?

One of the main benefits of using mocks and stubs is that they allow you to verify the behavior of the unit under test, by checking how it interacts with the dependencies. To verify mocks and stubs, you can use the assert methods provided by the unittest framework, or the methods and attributes of the mock or stub objects. For example, you can use assert_called, assert_called_once, assert_called_with, assert_called_once_with, assert_any_call, assert_has_calls, assert_not_called, and assert_call_count to check the number, order, and arguments of the calls made to the mock or stub object. You can also use assert_called, return_value, side_effect, and called to inspect the state of the mock or stub object.

4 How to debug mocks and stubs?

Sometimes, your tests may fail or behave unexpectedly because of a problem with your mocks or stubs. For example, you may have created a mock or stub object with the wrong configuration, or you may have missed or mismatched some calls to the mock or stub object. To debug mocks and stubs, you can use some techniques and tools, such as logging, breakpoints, print statements, or mock.call_args_list. Logging can help you track the execution flow and the calls made to the mock or stub object. Breakpoints can help you pause the execution and inspect the state of the mock or stub object. Print statements can help you print the values of the mock or stub object or its attributes. mock.call_args_list can help you see the list of call arguments made to the mock or stub object.

5 How to improve mocks and stubs?

While mocks and stubs are useful for unit testing, they also have some drawbacks and limitations. For example, they may introduce complexity and maintenance overhead, they may hide some errors or bugs in the real dependencies, and they may not reflect the actual behavior or state of the real dependencies. To improve mocks and stubs, you can follow some best practices and guidelines, such as using them sparingly and only when necessary, using them consistently and clearly, using them with realistic and meaningful data, using them with verification and not just for setup, and using them with tools that can generate or validate them automatically.