TDD — Red-Green-Refactor cycle

4.5.14 · Coding › Software Engineering

WHAT is TDD?

Teen states:

WHY do it this way?

HOW the cycle runs (step by step)

Worked Example 1 — Scratch se FizzBuzz banana (tests se code ka derivation)

Hum implementation ko purely Red-Green-Refactor follow karke derive karte hain.

Cycle 1 — RED

def test_one_returns_1():
    assert fizzbuzz(1) == "1"

Yeh step kyun? Sabse chhota possible behavior. Run karo to fail karta hai: fizzbuzz exist nahi karta (sahi reason se Red — stub karne ke baad).

Cycle 1 — GREEN

def fizzbuzz(n):
    return "1"      # fake it — hardcoded!

Yeh step kyun? Pass karne ke liye minimum. Hardcoding intentional hai — yeh agli test ko generalization drive karne par majboor karta hai.

Cycle 2 — RED

def test_two_returns_2():
    assert fizzbuzz(2) == "2"

Kyun? Yeh humein "1" hardcode karna band karne par majboor karta hai.

Cycle 2 — GREEN

def fizzbuzz(n):
    return str(n)

Kyun? Ab sirf utna hi generalize kiya jितना chahiye. Dono tests pass hain.

Cycle 3 — RED → GREEN (Fizz rule)

def test_three_returns_Fizz():
    assert fizzbuzz(3) == "Fizz"

def fizzbuzz(n):
    if n % 3 == 0:
        return "Fizz"
    return str(n)

Cycle 4 — Buzz add karo, phir FizzBuzz, har ek apni failing test se driven. Saare tests green hone ke baad:

REFACTOR

def fizzbuzz(n):
    out = ""
    if n % 3 == 0: out += "Fizz"
    if n % 5 == 0: out += "Buzz"
    return out or str(n)

Yeh step kyun? Branchy duplication hata di. Koi naya test nahi chahiye — behavior identical hai; tests abhi bhi pass hain, jo prove karta hai ki refactor safe hai.

Worked Example 2 — Test se bug fix drive karna

Tumne ek bug pakda: divide(10, 0) ungracefully crash karta hai.

RED — bug ko test ki tarah reproduce karo

def test_divide_by_zero_raises_value_error():
    with pytest.raises(ValueError):
        divide(10, 0)

Kyun? Failing test bug ko precisely capture karta hai. Yeh abhi fail hota hai (ZeroDivisionError raise karta hai, ValueError nahi).

GREEN

def divide(a, b):
    if b == 0:
        raise ValueError("cannot divide by zero")
    return a / b

Kyun? Minimal guard. Test pass ho jaata hai — aur yeh suite mein hamesha ke liye regression guard ki tarah rehta hai taaki bug kabhi silently wapas na aa sake.

Economics — Chhote cycles kyun matter karte hain

Flashcards

Connections

• Unit Testing — TDD unit tests pe built hai feedback unit ki tarah.
• Refactoring — teesra phase; safe tabhi jab tests green hain.
• Red-Green-Refactor vs BDD - Behavior Driven Development — BDD, TDD ko user-story language tak le jaata hai.
• Regression Testing — har TDD test ek regression guard ban jaata hai.
• Code Coverage — TDD ka side effect hai, goal nahi.
• Continuous Integration — green test suite merge karne ka gate hai.
• SOLID Principles — testable code naturally loose coupling ki taraf trend karta hai.
• Test Doubles - Mocks Stubs Fakes — unit under test ko isolate karne ke liye use hote hain.

Concept Map