Introduction to Go Testing

In this lesson, explore introduction to go testing through one small Go program, and connect its output to the language rules behind it.

Go has a built-in testing package and a go test command. Place tests in files ending with _test.go; these files compile only for tests, not normal go run.

A test should state an input, an expected result and what to report when behavior differs. Keep tests deterministic and small before layering in external dependencies.

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

Actual test file

Place this code in add_test.go, alongside your production add function:

package main
import "testing"
func TestAdd(t *testing.T) {
    if got := add(2, 3); got != 5 { t.Fatalf("add(2,3)=%d; want 5", got) }
}

From your module directory run go test ./.... See Add a test.

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 add(a, b int) int { return a + b }
func main() {
	got, want := add(2, 3), 5
	fmt.Println("Example passes:", got == want)
}

Output

Example passes: true

The executable checks an example expectation to introduce the assertion idea. In real Go testing, move that assertion into TestAdd in add_test.go and run go test.

• A main.go printout is not a substitute for a test that the go test command runs.
• Tests should report failures with useful input and expected-versus-actual values.

Create a TestAdd function in add_test.go with three different inputs.

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

You have practiced Introduction to Go Testing 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.