Code Smell 254 — Mystery Guest

You assert that something happened, but why?

TL;DR: Be explicit when creating tests to ensure clarity and maintainability

Problems

• Readability
• Coupling to external databases, global state or singletons, static methods or external services
• Maintenance Difficulty
• Debugging Complexity
• Hidden Dependencies

Solutions

1. Be Explicit
2. Inline the setup
3. Use dependency Injection
4. Use mocking with caution

Context

Your test depends on external data or configurations not immediately visible within the test itself.

This obscures the test’s setup, making it difficult for someone reading it to understand what is being tested and why it might fail.

Every test case should have three stages:

1. Setup: Initialize and configure everything needed for the test.
2. Excercise: Execute the code being tested.
3. Assert: Verify the expected outcome.

All of them must be explicit

Sample Code

Wrong

@Test
void shouldReturnAnswerWhenAnswerExists() {
    User answer = KnowledgeRepository.findAnswerToQuestion(42);
    assertNotNull(answer);
}

Right

@Test
void shouldReturnAnswerWhenAnswerExists() {
    KnowledgeRepository knowledgeRepository = 
        new InMemoryKnowledgeRepository();
    Answer expectedAnswer = new Answer(42, "The Ultimate");
    knowledgeRepository.save(expectedAnswer);

    Answer actualAnswer = answerRepository.findAnswerToQuestion(42);
    assertNotNull(actualAnswer);
    assertEquals(expectedAnswer, actualAnswer);
}

Detection

[X] Manual

You can detect this smell by looking for tests that do not clearly show their setup steps or rely heavily on external configurations.

Tags

• Test Smells

Level

[x] Intermediate

AI Generation

AI-generated code often avoids this smell due to the tendency to create small, isolated examples.

AI Detection

Most AI Detectors fail to identify this as a problem unless you point it out explicitly.

Conclusion

This code smell is especially prevalent in legacy codebases or when consistent testing practices are lacking.

You need to be explicit of the environment since tests must always be in “full environmental control”

Relations

Code Smell 17 — Global Functions

Code Smell 32 — Singletons

Code Smell 18 — Static Functions

Code Smell 30 — Mocking Business

More Info

Transform Your Craft with TDD

Coupling: The one and only software design problem

Disclaimer

Code Smells are my opinion.

Credits

Photo by Brands&People on Unsplash

Science is what we understand well enough to explain to a computer, Art is all the rest

Donald Knuth

Software Engineering Great Quotes

This article is part of the CodeSmell Series.

How to Find the Stinky parts of your Code