Unit Testing Tenets

Tres Seaver’s unit testing tenets.

Goals

Unit tests should:

• be as simple as possible while still exercising the app under test.
• run as quickly as possible to encourage running them frequently.
• avoid couplings with other tests, or with parts of the codebase they are not responsible for testing.
• be idempotent.

Tests should not:

• replace narrative or API documentation.

Rules

• Never import the module under test at test module scope.
• Each test case method tests Just One Thing.
• Test case names indicate what they test.
• Don’t share test fixtures between test modules.

Guidelines

• Minimize module-scope dependencies.
• Do as little work as possible in the execution path of a single test.
• Make fixtures as simple as possible.
• Use globals judiciously.
• Use stub/mock objects to clarify dependent contracts.

Conclusions

• Tests are straightforward to write.
• Excellent coverage.
• Easy to debug/diagnose.
• Quick to run.
• Clarifies thinking about the code and its dependencies.

Demonstration

Let’s go back and improve the tests we wrote using Tres’ tenets.

What Do We Want To Fix

• Bad test practices (importing things under-test at module scope).
• Bad/missing stubs or mocks.
• Bad fixtures.
• Misuse of globals.

API Talk (Tangent)

• Avoid Global State When Possible
• Avoid Designing for Convenience (Don’t Give In To import Fascination)
• Composition Usually Beats Inheritance
• Avoid Using Others’ Code That Breaks These Rules