Go Foundations·Clarity, then concurrency

Testing and benchmarks

Write table tests, run the race detector and measure performance with benchmarks.

Beginner 13 min read Lesson 4 of 300 Challenge included

What you will be able to do

Write test functions that take *testing.T and report failures with t.Errorf.
Drive many cases through one loop with a table test.
Run go test -race to catch data races.
Measure hot paths with a Benchmark function.

In plain English

A test in Go is a function that starts with Test and takes a *testing.T value, which reports failures with methods like t.Errorf. A table test is a slice of cases that you loop over, so many inputs share one piece of test logic. The go test command runs tests in a package, and adding -race enables the race detector, which finds data races where two concurrent operations touch the same value without coordination. A benchmark is a function starting with Benchmark that measures how long an operation takes by running it many times.

Step by step

1Write a Test function that accepts *testing.T.
2Build a slice of table cases with inputs and expected results.
3Loop over cases and fail with t.Errorf when a result is wrong.
4Run go test and go test -race from the package directory.
5Add a Benchmark function for code you want to measure.

add_test.goGo

package lessons

import "testing"

func Add(a, b int) int {
    return a + b
}

func TestAdd(t *testing.T) {
    cases := []struct {
        name string
        a    int
        b    int
        want int
    }{
        {"positive", 2, 3, 5},
        {"zero", 0, 0, 0},
        {"negative", -1, 1, 0},
    }

    for _, c := range cases {
        t.Run(c.name, func(t *testing.T) {
            got := Add(c.a, c.b)
            if got != c.want {
                t.Errorf("Add(%d, %d) = %d, want %d", c.a, c.b, got, c.want)
            }
        })
    }
}

func BenchmarkAdd(b *testing.B) {
    for i := 0; i < b.N; i++ {
        Add(2, 3)
    }
}

Common mistakes

Naming a test without the Test prefix means go test will silently skip it.
Calling t.Fatal in a table test without t.Run stops the whole test instead of just one case.
Running benchmarks with go test without the -bench flag will not execute them.
Skipping the race detector hides data races that only appear under concurrency.

Your turn

Write a table test

Medium

Write a TestDouble that uses a table of cases to check Double(n) returns 2n for at least three inputs, reporting failures with t.Errorf.

GO · starter

func Double(n int) int {
    return n * 2
}

func TestDouble(t *testing.T) {
}

Check your understanding before moving on.

Knowledge checkQuestion 1

What does go test -race detect?

Select an answer to continue

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