Test-driven development (TDD) concept in Testing Fundamentals - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the first step in the TDD cycle?

In Test-driven development (TDD), which step comes first in the cycle?

A Write the minimal code to pass the test

B Refactor the existing code to improve structure

C Write a failing test that defines a new function or feature

D Run all tests to ensure everything passes

💡 Hint

Think about what guides the development in TDD.

The TDD cycle starts by writing a test that fails because the feature is not implemented yet. This test defines what the code should do.

❓ Predict Output

intermediate

What is the output of this TDD test run?

Given the following Python test code using unittest, what will be the test result when run the first time?

Testing Fundamentals

import unittest

def add(a, b):
    pass  # Not implemented yet

class TestAdd(unittest.TestCase):
    def test_add_positive(self):
        self.assertEqual(add(2, 3), 5)

if __name__ == '__main__':
    unittest.main()

A Test passes successfully

B Test fails with AssertionError

C SyntaxError due to missing return

D TypeError because add() returns None

💡 Hint

What does the add function return currently?

The add function is not implemented and returns None by default. The test expects 5, so the assertion fails.

❓ assertion

advanced

Which assertion best fits TDD for checking a list contains a new item?

You are writing a test first in TDD to check that a function adds an item to a list. Which assertion is the best choice?

A self.assertIsNotNone(result_list)

B self.assertEqual(result_list[-1], 'item')

C self.assertListEqual(result_list, ['item'])

D self.assertTrue('item' in result_list)

💡 Hint

Focus on what exactly you want to verify about the list.

Using assertTrue with 'in' checks that the item is present anywhere in the list, which is the most direct and flexible check for this case.

🔧 Debug

advanced

Why does this TDD test keep passing incorrectly?

In TDD, you wrote this test and function. The test passes but the function is clearly wrong. Why?

Testing Fundamentals

def is_even(n):
    return True

def test_is_even():
    assert is_even(3) == False
    assert is_even(4) == True

A The test passes because assert statements are not executed when Python runs with optimizations

B The function always returns True, so the test should fail but does not

C The test uses assert statements outside a test framework, so failures are ignored

D The test is not run because it lacks a test framework decorator

💡 Hint

Consider how Python treats assert statements when run with certain flags.

When Python runs with the -O (optimize) flag, assert statements are skipped, so the test does not fail even if the function is wrong.

❓ framework

expert

Which test runner command fits TDD workflow for Python unittest?

In a TDD workflow using Python's unittest, which command runs tests and stops at the first failure to help quick feedback?

A python -m unittest --failfast

B python -m unittest -f

C python -m unittest -v

D python -m unittest discover

💡 Hint

Look for the option that stops testing on first failure.

The --failfast option stops the test run immediately after the first failure, which is useful in TDD for fast feedback.

Practice

1. What is the main idea behind Test-driven development (TDD)?

easy

A. Write tests only for completed features

B. Write code first, then write tests after

C. Write tests before writing the actual code

D. Write tests only for bugs found in production

Solution

1. Step 1: Understand TDD process

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

2. Step 2: Compare options with TDD definition

   Only Write tests before writing the actual code correctly states writing tests before code.

3. Final Answer:

   Write tests before writing the actual code -> Option C

4. Quick Check:

   TDD = Write tests first [OK]

Hint: Remember: Tests come first in TDD [OK]

Common Mistakes:

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

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

easy

A. Write test, write code, refactor

B. Refactor, write test, write code

C. Write code, write test, refactor

D. Write code, refactor, write test

Solution

1. Step 1: Recall the Red-Green-Refactor cycle

   The TDD cycle is: write a failing test (Red), write code to pass test (Green), then refactor.

2. Step 2: Match cycle to options

   Write test, write code, refactor matches the correct order: write test, write code, refactor.

3. Final Answer:

   Write test, write code, refactor -> Option A

4. Quick Check:

   TDD cycle = Test, Code, Refactor [OK]

Hint: Think: Red (test), Green (code), Refactor [OK]

Common Mistakes:

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

3. Consider this TDD workflow:

1. Write a test that fails
2. Write code to pass the test
3. Refactor the code
4. Repeat for next feature

What will happen if you skip step 1 and write code first?

medium

A. Tests will automatically pass without issues

B. The code will be bug-free by default

C. Refactoring will be unnecessary

D. You may write code that does not meet requirements

Solution

1. Step 1: Understand the role of failing tests in TDD

   Failing tests define what the code should do. Skipping them risks missing requirements.

2. Step 2: Analyze consequences of skipping step 1

   Without tests first, code may not meet expected behavior, causing bugs or missing features.

3. Final Answer:

   You may write code that does not meet requirements -> Option D

4. Quick Check:

   Skip test first = Risk wrong code [OK]

Hint: Tests guide code; skip tests, code may miss goals [OK]

Common Mistakes:

• Assuming code is always correct without tests
• Thinking refactoring fixes missing tests
• Believing tests are optional in TDD

4. You wrote a test first but it never fails when you run it initially. What is the likely problem?

medium

A. The test is not testing anything or is incorrect

B. The code is perfect and needs no changes

C. Refactoring was done before writing the test

D. The test runner is broken and skips tests

Solution

1. Step 1: Understand expected test behavior in TDD

   Tests should fail first to show missing functionality.

2. Step 2: Interpret why test never fails initially

   If test never fails, it likely does not check anything meaningful or has errors.

3. Final Answer:

   The test is not testing anything or is incorrect -> Option A

4. Quick Check:

   Failing test first means test must fail initially [OK]

Hint: Failing test first means test must fail initially [OK]

Common Mistakes:

• Assuming code is already perfect
• Ignoring test correctness
• Blaming test runner without checking test code

5. You are using TDD to develop a feature that calculates discounts. You write a test for a 10% discount but forget to handle zero or negative prices. What is the best TDD approach to fix this?

hard

A. Ignore zero and negative prices as invalid inputs

B. Write new failing tests for zero and negative prices, then update code

C. Refactor existing test to include zero and negative prices without new tests

D. Write code first to handle zero and negative prices, then write tests

Solution

1. Step 1: Identify missing test cases in TDD

   TDD requires writing tests for all expected cases, including edge cases like zero and negative prices.

2. Step 2: Apply TDD cycle to add missing cases

   Write new failing tests for these cases, then write code to pass them, then refactor.

3. Final Answer:

   Write new failing tests for zero and negative prices, then update code -> Option B

4. Quick Check:

   Add tests for new cases before coding [OK]

Hint: Add tests for all cases before coding [OK]

Common Mistakes:

• Coding before writing tests for new cases
• Ignoring edge cases in tests
• Trying to refactor tests without adding new ones