Should we combine tests?
Combining the two scenarios — basic cost calculation and applying a "Buy 2 Get 1 Free" offer — into a single test is generally not recommended in the context of Test-Driven Development (TDD). Here's why:
Single Responsibility Principle: Each test should ideally test only one behavior or aspect of the system. The first test checks the basic calculation of total cost without any offers, while the second test verifies the application of a specific offer. Combining them would mean testing two different behaviors in one test, which can complicate the test and make it harder to understand and maintain.
Clarity and Debugging: If a combined test fails, it could be due to a problem in the basic calculation logic, the offer application logic, or the interaction between the two. This ambiguity makes debugging more difficult. Separate tests ensure that if a test fails, it's clear what specific part of the functionality is not working as expected.
Scope of Change Impact: If the business logic for offers changes in the future, it would only impact the tests specifically designed for those offers. Similarly, changes in the basic cart calculation should only impact the basic calculation tests. Combining the tests means that changes in one aspect could require unnecessary modifications to a test that also covers other aspects.
Readability and Documentation: Separate tests serve as better documentation for how different parts of the system are supposed to work. They clearly articulate the expected behavior of each aspect of the system in isolation.
In summary, while it might seem efficient to combine tests to cover multiple scenarios at once, it's generally better, especially in TDD, to keep tests focused and specific. This approach leads to a suite of tests that are easier to understand, maintain, and debug.
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