Writing Unit Tests

In this lesson, explore writing unit tests through one small Go program, and connect its output to the language rules behind it.

A unit test checks one small behavior under controlled conditions. Functions named TestXxx(t *testing.T) in _test.go files are discovered automatically.

Use t.Errorf to report a failure and continue checking; use t.Fatalf when continuing would not make sense.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

unit_test.go

package main
import "testing"
func TestIsEven(t *testing.T) {
    if !isEven(2) { t.Error("2 should be even") }
    if isEven(3) { t.Error("3 should be odd") }
}

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func isEven(n int) bool { return n%2 == 0 }
func main() {
	fmt.Println("2 even:", isEven(2))
	fmt.Println("3 even:", isEven(3))
}

Output

2 even: true
3 even: false

The function reports true for 2 and false for 3. A test can assert both expectations and report a precise error when a regression occurs.

• Test exported and unexported behavior through useful package boundaries.
• Avoid brittle assertions against incidental implementation details.

Write TestIsEven to check 0, 2, 3 and -4.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Writing Unit Tests in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.