Test-driven development (TDD) in Software Engineering - Step-by-Step Execution

Concept Flow - Test-driven development (TDD)

Write a Test

↓

Run Test -> Fails

↓

Write Code to Pass Test

↓

Run Test -> Passes

↓

Refactor Code

↓

Run Test -> Passes

↓

Repeat for Next Feature

TDD is a cycle where you first write a test that fails, then write code to pass it, and finally refactor while keeping tests green.

Execution Sample

Software Engineering

def add(a, b):
    return a + b

# Test: assert add(2, 3) == 5

This code defines a simple add function and a test that checks if adding 2 and 3 equals 5.

Analysis Table

Step	Action	Test Result	Code State	Next Step
1	Write test for add(2,3)==5	Fail (no code yet)	No add function	Write add function
2	Write add function returning a+b	Pass	add function implemented	Refactor if needed
3	Run test after refactor	Pass	Code clean and working	Write next test
4	Write test for add(-1,1)==0	Fail (new test)	Code unchanged	Write code to pass test
5	Update add function (if needed)	Pass	Function handles new case	Repeat cycle
6	Cycle continues for new features	Tests guide development	Code evolves safely	Continue TDD loop

💡 Testing stops when all tests pass and code is refactored cleanly for current features.

State Tracker

Variable	Start	After Step 2	After Step 5	Final
add function	undefined	defined as return a+b	same function, verified for new cases	stable and tested function

Key Insights - 3 Insights

Why do we write tests before writing the actual code?

Writing tests first ensures you know exactly what the code should do. As shown in execution_table step 1, the test fails initially because the code doesn't exist yet, guiding development.

What if the test never passes?

If the test never passes, it means the code doesn't meet the requirement. You must fix the code until the test passes, as shown in steps 2 and 5 where code changes lead to passing tests.

Why do we refactor after tests pass?

Refactoring cleans the code without changing behavior. Tests ensure the code still works after refactor, as seen in step 3 where tests pass after cleaning code.

Visual Quiz - 3 Questions

Look at the execution_table, at which step does the test first pass?

A Step 4

B Step 1

C Step 2

D Step 5

💡 Hint

Check the 'Test Result' column for the first 'Pass' entry.

According to variable_tracker, what is the state of the add function after step 5?

A Same function, verified for new cases

B Undefined

C Defined but untested

D Removed

💡 Hint

Look at the 'After Step 5' column for 'add function' in variable_tracker.

If the test fails at step 4, what should be the next action according to the concept_flow?

A Write a new test

B Write code to pass the test

C Refactor code

D Stop development

💡 Hint

Refer to the flow after 'Run Test -> Fails' which leads to 'Write Code to Pass Test'.

Concept Snapshot

Test-driven development (TDD) cycle:
1. Write a failing test.
2. Write code to pass the test.
3. Run tests to confirm pass.
4. Refactor code safely.
5. Repeat for new features.
Tests guide and protect code quality.

Full Transcript

Test-driven development (TDD) is a software practice where you first write a test that fails because the code does not exist yet. Then you write just enough code to make the test pass. After that, you refactor the code to improve its structure without changing behavior. You run tests after each step to ensure correctness. This cycle repeats for every new feature or change, ensuring code is always tested and reliable.