Mastering Go Testing: From Basics to Subtests in 3 Minutes

Source: Dev.to

Level 1: The Basic Test

A test in Go is just a function that starts with Test and takes a *testing.T.

func TestAdd(t *testing.T) {
    result := Add(2, 3)
    expected := 5

    if result != expected {
        t.Errorf("Add(2, 3) = %d; want %d", result, expected)
    }
}

We don’t need external assertion libraries—simple if statements and t.Errorf are enough.

Level 2: Table‑Driven Tests

Copy‑pasting the same test for different inputs is tedious and error‑prone.
Table‑driven tests let you define a slice of test cases and iterate over them.

func TestAdd(t *testing.T) {
    tests := []struct {
        a, b     int
        expected int
    }{
        {2, 3, 5},
        {0, 0, 0},
        {-1, 1, 0},
    }

    for _, tt := range tests {
        result := Add(tt.a, tt.b)
        if result != tt.expected {
            t.Errorf("Add(%d, %d) = %d; want %d", tt.a, tt.b, result, tt.expected)
        }
    }
}

Adding a new case is just another line in the slice.

Level 3: Subtests with t.Run

Subtests give each case a distinct name, improving output and debugging.

func TestAdd(t *testing.T) {
    tests := []struct {
        name     string // New!
        a, b     int
        expected int
    }{
        {"positive numbers", 2, 3, 5},
        {"zeros", 0, 0, 0},
        {"negative", -1, 1, 0},
    }

    for _, tt := range tests {
        // t.Run creates a subtest
        t.Run(tt.name, func(t *testing.T) {
            result := Add(tt.a, tt.b)
            if result != tt.expected {
                t.Errorf("got %d, want %d", result, tt.expected)
            }
        })
    }
}

Why is this better?

• Clear output / easier debugging – failures show the subtest name, e.g. TestAdd/positive numbers.

• Granular control – run a single subtest from the command line:

  go test -run TestAdd/zeros

• Parallelism – invoke t.Parallel() inside the subtest to run cases concurrently.

Summary

• Basics: Use t.Errorf to report failures.
• Table‑driven: Organize test data in a []struct.
• Subtests: Use t.Run for better reporting, selective execution, and parallelism.

Happy testing!