Test-driven development (TDD) in Software Engineering - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is Test-driven development (TDD)?

TDD is a software development approach where tests are written before writing the actual code. It helps ensure the code works as expected by guiding development through tests.

beginner

What are the three main steps in the TDD cycle?

The three steps are: 1) Write a failing test, 2) Write code to pass the test, 3) Refactor the code to improve it while keeping tests passing.

intermediate

Why is writing tests first beneficial in TDD?

Writing tests first helps clarify what the code should do, prevents overcoding, and ensures that every piece of code is tested from the start.

intermediate

How does TDD improve software quality?

TDD improves quality by catching bugs early, encouraging simple design, and providing a safety net for future changes through automated tests.

beginner

What is refactoring in the TDD process?

Refactoring means improving the code's structure and readability without changing its behavior, done after tests pass to keep code clean and maintainable.

What is the first step in the TDD cycle?

A Refactor the code

B Write the full code

C Run all tests

D Write a failing test

In TDD, you start by writing a test that fails because the feature is not implemented yet.

Why do developers refactor code after writing tests in TDD?

A To improve code without changing behavior

B To add new features

C To remove tests

D To make tests fail

Refactoring improves code quality and readability without changing what the code does.

Which of the following is NOT a benefit of TDD?

A Faster initial coding without tests

B Early bug detection

C Clear requirements

D Better code design

TDD requires writing tests first, so initial coding is not faster but leads to better quality.

In TDD, what does a passing test indicate?

A The test is incorrect

B The code meets the test requirements

C The code needs more features

D The test should be deleted

A passing test means the code behaves as expected for that test case.

How does TDD help with future code changes?

A By removing old code

B By delaying testing until the end

C By providing automated tests as safety checks

D By avoiding writing tests

Automated tests created during TDD help ensure future changes do not break existing functionality.

Explain the Test-driven development (TDD) cycle and why each step is important.

Think about how tests guide coding and how refactoring keeps code clean.

Describe the main benefits of using TDD in software development.

Consider how tests help both during and after development.

Practice

1. What is the main purpose of Test-driven development (TDD)?

easy

A. To write tests before writing the actual code

B. To write code first and then tests later

C. To avoid writing any tests

D. To write documentation before coding

Solution

1. Step 1: Understand TDD process

   TDD means writing tests before the actual code to guide development.

2. Step 2: Identify the main goal

   The main goal is to ensure code correctness by writing tests first.

3. Final Answer:

   To write tests before writing the actual code -> Option A

4. Quick Check:

   TDD = Write tests first [OK]

Hint: Remember: TDD starts with tests, not code [OK]

Common Mistakes:

• Thinking code is written before tests
• Confusing TDD with writing documentation
• Believing tests are optional in TDD

2. Which of the following is the correct order of steps in the TDD cycle?

easy

A. Write code, write test, refactor

B. Write test, write code, refactor

C. Refactor, write code, write test

D. Write test, refactor, write code

Solution

1. Step 1: Recall TDD cycle steps

   The TDD cycle is: write a failing test, write code to pass it, then refactor.

2. Step 2: Match the correct sequence

   Write test, write code, refactor matches this order exactly.

3. Final Answer:

   Write test, write code, refactor -> Option B

4. Quick Check:

   TDD cycle = Test -> Code -> Refactor [OK]

Hint: TDD always starts with a failing test [OK]

Common Mistakes:

• Writing code before tests
• Refactoring before writing code
• Mixing the order of steps

3. Consider this TDD scenario:
1. Write a test that expects function add(2, 3) to return 5.
2. Implement add function as def add(a, b): return a - b
3. Run the test.
What will be the test result?

medium

A. Syntax error in function definition

B. Test passes successfully

C. Test fails because the function returns wrong result

D. Test is skipped automatically

Solution

1. Step 1: Analyze the function implementation

   The function subtracts b from a instead of adding them.

2. Step 2: Compare expected and actual results

   Calling add(2, 3) returns -1, but test expects 5, so test fails.

3. Final Answer:

   Test fails because the function returns wrong result -> Option C

4. Quick Check:

   Wrong function logic = Test fails [OK]

Hint: Check if function logic matches test expectation [OK]

Common Mistakes:

• Assuming test passes despite wrong logic
• Confusing syntax error with logic error
• Thinking tests skip automatically

4. You wrote a test first but it never fails even before implementing the code. What is the likely problem?

medium

A. Test is not written correctly to check the feature

B. Code is already perfect and needs no changes

C. Test framework is broken and skips tests

D. You forgot to run the test

Solution

1. Step 1: Understand TDD failing test requirement

   In TDD, the first test should fail because code is not implemented yet.

2. Step 2: Identify why test never fails

   If test never fails, it likely does not check the feature properly or has wrong assertions.

3. Final Answer:

   Test is not written correctly to check the feature -> Option A

4. Quick Check:

   Failing test means correct test setup [OK]

Hint: First test must fail to guide development [OK]

Common Mistakes:

• Assuming code is perfect without implementation
• Blaming test framework without checking test code
• Forgetting to run tests

5. You want to use TDD to develop a function that returns only even numbers from a list. Which sequence of steps best follows TDD principles?

hard

A. Write code and tests simultaneously, then refactor

B. Write code to filter evens, then write tests to check it

C. Write tests after finishing all code

D. Write a test for filtering evens, write code to pass test, then refactor code

Solution

1. Step 1: Recall TDD workflow

   TDD requires writing a failing test first, then code to pass it, then refactoring.

2. Step 2: Match steps to TDD

   Write a test for filtering evens, write code to pass test, then refactor code follows this exact sequence for developing the function.

3. Final Answer:

   Write a test for filtering evens, write code to pass test, then refactor code -> Option D

4. Quick Check:

   TDD = Test -> Code -> Refactor [OK]

Hint: Always start with a test that fails for new feature [OK]

Common Mistakes:

• Writing code before tests
• Writing tests after code completion
• Mixing test and code writing steps