Go でコマンド ライン フラグの検証を単体テストする方法

<code class="go">var formatType string

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

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

    flag.StringVar(&amp;formatType, "format", defaultFormat, formatUsage)
    flag.StringVar(&amp;formatType, "f", defaultFormat, formatUsage+" (shorthand)")
}</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 &amp;&amp; *f != "text" {
        return errors.New("format flag already set")
    }
    if value != "text" &amp;&amp; value != "json" &amp;&amp; value != "hash" {
        return errors.New("Invalid Format Type")
    }
    *f = formatType(value)
    return nil
}</code>

<code class="go">func TestValidateFormatFlag(t *testing.T) {
    testCases := []struct {
        input    string
        expected error
    }{
        {"text", nil},
        {"json", nil},
        {"hash", nil},
        {"invalid", errors.New("Invalid Format Type")},
    }

    for _, tc := range testCases {
        args := []string{"-format", tc.input}
        os.Args = args

        typeFlag = "text"
        flag.CommandLine = flag.NewFlagSet("test", flag.ContinueOnError)
        flag.Var(&amp;typeFlag, "format", "Format type. Must be \"text\", \"json\" or \"hash\". Defaults to \"text\".")
        if err := flag.CommandLine.Parse(args[1:]); err != nil {
            t.Errorf("Error parsing flags: %v", err)
        }

        if tc.expected != nil && err == nil {
            t.Errorf("Expected error: %v, got nil", tc.expected)
        } else if tc.expected == nil && err != nil {
            t.Errorf("Unexpected error: %v", err)
        }
    }
}</code>