tdd-workflow

TDD Workflow

In test-driven development the test comes before the code. You describe the behavior you want as an executable expectation, watch it fail, then write just enough to satisfy it. The test becomes the specification, and a passing suite becomes the proof.

The loop

  ┌─────────────────────────────────────────────┐
  │  RED      write a test; run it; watch it fail │
  │   ↓                                           │
  │  GREEN    write the least code that passes it │
  │   ↓                                           │
  │  REFACTOR tidy up while every test stays green│
  │   ↓                                           │
  └──── repeat for the next behavior ─────────────┘

Three constraints keep the loop honest:

1. Add production code only in response to a test that is currently failing.
2. Write no more of a test than it takes to show that failure.
3. Write no more code than it takes to turn that test green.

RED — start with a failing test

Express what the code should do, never how it does it.

Aim at	Example name
Core behavior	adds two positive numbers
Boundaries	returns zero for an empty list
Failure modes	raises on a negative quantity

Ground rules: the test must fail before you write any implementation, its name should read as a sentence about behavior, and each test should pin down a single idea.

# RED — this fails because cart_total does not exist yet
def test_cart_total_sums_line_items():
    cart = Cart(items=[Item(price=300), Item(price=150)])
    assert cart.total() == 450

GREEN — do the minimum to pass

Write the plainest thing that turns the bar green. No speculative features, no tuning.

Principle	What it means here
Avoid speculation	If a test does not demand it, do not build it
Plainest solution	The most obvious code that satisfies the assertion
Defer performance	Make it correct now; make it fast later, if measured

# GREEN — just enough to satisfy the test
class Cart:
    def __init__(self, items):
        self.items = items
    def total(self):
        return sum(item.price for item in self.items)