Welcome to the first lesson of our course on Test Driven Development (TDD) in Go using Testify. TDD is an iterative software development process where tests are written prior to developing the actual functionality. This approach helps developers focus on the requirements first, leading to more reliable and maintainable code.

In this lesson, we'll introduce you to the essential elements of TDD, including the Red-Green-Refactor cycle, which serves as the core structure of this methodology. We'll be utilizing tools specially suited for Go: Testify, a popular testing library that is robust and integrates well with Go. These tools are excellent for defining and running tests in Go. Let's start by investigating TDD’s core components with a hands-on example.

The TDD process begins with writing a test that fails, marking the "Red" phase. This step allows you to crystallize what the code should achieve before writing the actual implementation. Let's write a test for a Sum function that should eventually add two numbers.

Create a file named math_test.go in the tests directory:

package math

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

func TestSum(t *testing.T) {
    math := new(Math)
    assert.Equal(t, 5, math.Sum(2, 3))
}

This test script:

• Uses Testify to assert equality between expected and actual results.
• Instantiates a Math struct.
• Calls the Sum method and checks if the result equals 5.

Attempt to run this test although the Math struct and Sum function are not yet implemented. It will result in a compilation error, reflecting the "Red" stage, signaling the requirement for implementation.

Expected output:

# codesignal.com/testing [codesignal.com/testing.test]
./math_test.go:9:17: undefined: Math
FAIL    codesignal.com/testing [build failed]
FAIL

This is a normal failure, illustrating that our test is effectively identifying unimplemented features.

Our next objective is to write the simplest code possible to make the test pass — the "Green" step. In TDD, it means implementing minimal functionality to satisfy the test conditions. Let’s define the Sum method in a new file math.go under the src directory:

package math

type Math struct{}

func (*Math) Sum(a, b int) int {
    return 5
}

This version seems superficial since it doesn't actually add two numbers, but it highlights the TDD focus on passing the test with minimal implementation. By doing so, we've met the test condition.

Re-running the test will produce the following result:

ok      codesignal.com/testing  0.003s

Seeing the test pass confirms our code meets the given test scenario. Future test cases will guide us to refine the implementation.