3.4 Writing Your First Tests | Errors, Testing & Confidence

Test File Convention

Tests live next to the code they test, in files ending with _test.go:

config/
├── config.go       # your code
├── config_test.go  # tests for config.go

Basic Test

// config_test.go
package config

import "testing"

func TestParsePort(t *testing.T) {
    port, err := ParsePort("8080")
    if err != nil {
        t.Fatalf("unexpected error: %v", err)
    }
    if port != 8080 {
        t.Errorf("got %d, want 8080", port)
    }
}

func TestParsePort_Invalid(t *testing.T) {
    _, err := ParsePort("not-a-number")
    if err == nil {
        t.Fatal("expected error for invalid port, got nil")
    }
}

t.Error vs t.Fatal

Method	Behavior
`t.Error(args...)`	Log failure, continue running this test
`t.Errorf(format, args...)`	Same, with formatting
`t.Fatal(args...)`	Log failure, stop this test immediately
`t.Fatalf(format, args...)`	Same, with formatting

Rule: Use t.Fatal when continuing would panic or be meaningless (e.g., a nil pointer). Use t.Error when you want to collect multiple failures.

Running Tests

go test ./...              # all tests, all packages
go test -v ./...           # verbose — see each test name
go test -run TestParsePort # run tests matching a pattern
go test -count=1 ./...     # bypass test cache
go test -race ./...        # detect data races