Comment tester unitairement la validation de l'énumération des indicateurs de ligne de commande dans Go ?

<code class="go">var formatType string

const (
    text = "text"
    json = "json"
    hash = "hash"
)

func init() {
    const (
        defaultFormat = "text"
        formatUsage   = "desired output format"
    )

    flag.StringVar(&formatType, "format", defaultFormat, formatUsage)
    flag.StringVar(&formatType, "f", defaultFormat, formatUsage+" (shorthand)")
}

func main() {
    flag.Parse()
}</code>

<code class="go">type formatType string

func (f *formatType) String() string {
    return fmt.Sprint(*f)
}

func (f *formatType) Set(value string) error {
    if len(*f) > 0 && *f != "text" {
        return errors.New("format flag already set")
    }
    if value != "text" && value != "json" && value != "hash" {
        return errors.New("Invalid Format Type")
    }
    *f = formatType(value)
    return nil
}</code>

<code class="go">package main

import "testing"

func TestFormatType(t *testing.T) {
    tests := []struct {
        args   []string
        expect string
    }{
        {[]string{"-format", "text"}, "text"},
        {[]string{"-f", "json"}, "json"},
        {[]string{"-format", "foo"}, "Invalid Format Type"},
    }

    for _, test := range tests {
        t.Run(test.args[0], func(t *testing.T) {
            flag.Parse()
            if typeFlag != test.expect {
                t.Errorf("Expected %s got %s", test.expect, typeFlag)
            }
        })
    }
}</code>