The TDD Mindset: Thinking in Tests with Go

In this lesson, we'll deepen our understanding of the Test-Driven Development (TDD) mindset by focusing on the Red-Green-Refactor cycle with a practical example centered on a CalculateTotal function. This example will guide you through the process of thinking in tests, prioritizing test writing, and leveraging TDD to enhance code clarity, reliability, and maintainability.

Using Go and Testify, we'll follow these steps:

• Begin with the Red phase by identifying and writing failing tests for the CalculateTotal function, which will compute the total price of items in a shopping cart.
• Move to the Green phase to implement the minimal code required to pass each test, ensuring that the CalculateTotal function behaves as expected.
• Enter the Refactor phase to improve the code structure and readability of CalculateTotal, employing idiomatic Go solutions while keeping tests green.
• Utilize Testify as a testing library to efficiently integrate the TDD approach within our Go project.

By working through this example, you'll gain practical experience with TDD principles and develop the CalculateTotal function in a way that showcases how TDD fosters code quality and robustness.

Let's begin by writing tests for a function named CalculateTotal, designed to compute the total price of items in a shopping cart. This is where you engage with the Red phase: Write a failing test.

Let's think about how to build a function that calculates lists of items. What should the interface be? How does the consumer of the code use it? These are the questions we think about first when we "think in tests." Here's one way we might think about it in Go.

package shoppingcart

import (
    "testing"
    "github.com/stretchr/testify/assert"
)

func TestCalculateTotal_ReturnsZeroForEmptySlice(t *testing.T) {
    result := CalculateTotal([]CartItem{})
    assert.Equal(t, 0.0, result)
}

Explanation:

• We know we want a function called CalculateTotal(), so we'll write a test that uses it.
• For now, we know that we want an empty slice as input to return 0, so we can write that test.
• The expectation is that these tests will initially fail, which is an integral part of the Red phase.

Running these tests confirms they fail, creating a clear path for subsequent development.